Command-not-found: Warn if a file exists but isn't executable

This simply adds a nice error so you can better see if your command
isn't available because it's not executable.

Fixes #8804.

share/config.fish

@@ -23,8 +23,15 @@ end
 #
 function __fish_default_command_not_found_handler
     printf "fish: Unknown command: %s\n" (string escape -- $argv[1]) >&2
+    for file in $PATH/$argv[1]
+        if test -e $file -a ! -x $file
+            printf (_ "fish: %s exists but isn't executable\n") (string escape -- $file) >&2
+            break
+        end
+    end
 end
 
+
 if not status --is-interactive
     # Hook up the default as the command_not_found handler
     # if we are not interactive to avoid custom handlers.

share/functions/fish_command_not_found.fish

@@ -13,6 +13,12 @@ end
 
 function __fish_default_command_not_found_handler
     printf "fish: Unknown command: %s\n" (string escape -- $argv[1]) >&2
+    for file in $PATH/$argv[1]
+        if test -e $file -a ! -x $file
+            printf (_ "fish: %s exists but isn't executable\n") (string escape -- $file) >&2
+            break
+        end
+    end
 end
 
 # If an old handler already exists, defer to that.

tests/checks/command-not-found.fish

@@ -32,4 +32,14 @@ echo $status
 # CHECKERR: { echo; echo }
 # CHECKERR: ^
 
+set -g PATH .
+echo banana > foobar
+foobar --banana
+# CHECKERR: fish: Unknown command: foobar
+# CHECKERR: fish: ./foobar exists but isn't executable
+# CHECKERR: checks/command-not-found.fish (line 37):
+# CHECKERR: foobar --banana
+# CHECKERR: ^
+
+
 exit 0