Improve error for redirections to invalid paths

This finds the first broken component, to help people figure out where
they misspelt something.

E.g.

```
echo foo >/usr/lob/systemd/system/machines.target.wants/var-lib-machines.mount
```

will now show:

```
warning: Path '/usr/lob' does not exist
```

which would help with seeing that it should be "/usr/lib".

src/io.cpp

@@ -7,6 +7,7 @@
 #include <fcntl.h>
 #include <stddef.h>
 #include <stdio.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include <cstring>
@@ -236,8 +237,32 @@ bool io_chain_t::append_from_specs(const redirection_spec_list_t &specs, const w
                     if ((oflags & O_EXCL) && (errno == EEXIST)) {
                         FLOGF(warning, NOCLOB_ERROR, spec.target.c_str());
                     } else {
-                        FLOGF(warning, FILE_ERROR, spec.target.c_str());
+                        if (should_flog(warning)) {
-                        if (should_flog(warning)) wperror(L"open");
+                            FLOGF(warning, FILE_ERROR, spec.target.c_str());
+                            auto err = errno;
+                            // If the error is that the file doesn't exist
+                            // or there's a non-directory component,
+                            // find the first problematic component for a better message.
+                            if (err == ENOENT || err == ENOTDIR) {
+                                auto dname = spec.target;
+                                struct stat buf;
+
+                                while (!dname.empty()) {
+                                    auto next = wdirname(dname);
+                                    if (!wstat(next, &buf)) {
+                                        if (!S_ISDIR(buf.st_mode)) {
+                                            FLOGF(warning, _(L"Path '%ls' is not a directory"), next.c_str());
+                                        } else {
+                                            FLOGF(warning, _(L"Path '%ls' does not exist"), dname.c_str());
+                                        }
+                                        break;
+                                    }
+                                    dname = next;
+                                }
+                            } else {
+                                wperror(L"open");
+                            }
+                        }
                     }
                     // If opening a file fails, insert a closed FD instead of the file redirection
                     // and return false. This lets execution potentially recover and at least gives

tests/checks/exec.fish

@@ -1,12 +1,12 @@
 #RUN: %fish %s
 exec cat <nosuchfile
 #CHECKERR: warning: An error occurred while redirecting file 'nosuchfile'
-#CHECKERR: open: No such file or directory
+#CHECKERR: warning: Path 'nosuchfile' does not exist
 echo "failed: $status"
 #CHECK: failed: 1
 not exec cat <nosuchfile
 #CHECKERR: warning: An error occurred while redirecting file 'nosuchfile'
-#CHECKERR: open: No such file or directory
+#CHECKERR: warning: Path 'nosuchfile' does not exist
 echo "neg failed: $status"
 #CHECK: neg failed: 0
 

tests/checks/pipestatus.fish

@@ -181,24 +181,24 @@ command true > /not/a/valid/path
 echo $pipestatus : $status
 #CHECK: 1 : 1
 #CHECKERR: warning: An error occurred while redirecting file '/not/a/valid/path'
-#CHECKERR: open: No such file or directory
+#CHECKERR: warning: Path '/not' does not exist
 
 # Here the first process will launch, the second one will not.
 command true | command true | command true > /not/a/valid/path
 echo $pipestatus : $status
 #CHECK: 0 0 1 : 1
 #CHECKERR: warning: An error occurred while redirecting file '/not/a/valid/path'
-#CHECKERR: open: No such file or directory
+#CHECKERR: warning: Path '/not' does not exist
 
 # Pipeline breaks do not result in dangling jobs.
 command true | command cat > /not/a/valid/path ; jobs
 #CHECKERR: warning: An error occurred while redirecting file '/not/a/valid/path'
-#CHECKERR: open: No such file or directory
+#CHECKERR: warning: Path '/not' does not exist
 #CHECK: jobs: There are no jobs
 
 # Regression test for #7038
 cat /dev/zero | dd > /not/a/valid/path
 echo 'Not hung'
 #CHECKERR: warning: An error occurred while redirecting file '/not/a/valid/path'
-#CHECKERR: open: No such file or directory
+#CHECKERR: warning: Path '/not' does not exist
 #CHECK: Not hung

tests/checks/redirect.fish

@@ -138,3 +138,7 @@ echo "/bin/echo pipe 12 <&12 12<&-" | source 12<&0
 #CHECK: pipe 10
 #CHECK: pipe 11
 #CHECK: pipe 12
+
+echo foo >/bin/echo/file
+#CHECKERR: warning: An error occurred while redirecting file '/bin/echo/file'
+#CHECKERR: warning: Path '/bin/echo' is not a directory

tests/pexpects/pipeline.py

@@ -37,6 +37,6 @@ expect_prompt("jobs: There are no jobs", unmatched="Should be no jobs")
 # Check that this weird invalid double-redirection doesn't crash fish.
 sendline("cat | cat </non/existent/file")
 expect_str("warning: An error occurred while redirecting file '/non/existent/file'")
-expect_str("open: No such file or directory")
+expect_str("warning: Path '/non' does not exist")
 send("\x04")
 expect_prompt()