Correct some comments and duplicative error messages

If we fail to create a pipe, we will report that fact in multiple places; remove
some redundant error reporting.

src/exec.rs

@@ -138,7 +138,6 @@ pub fn exec_job(parser: &Parser, job: &Job, block_io: IoChain) -> bool {
         if !p.is_last_in_job {
             let Ok(pipes) = make_autoclose_pipes() else {
                 FLOGF!(warning, "%ls", wgettext!(PIPE_ERROR));
-                perror("pipe");
                 aborted_pipeline = true;
                 abort_pipeline_from(job, i);
                 break;

src/fd_monitor.rs

@@ -63,7 +63,6 @@ impl FdEventSignaller {
         {
             // Implementation using pipes.
             let Ok(pipes) = make_autoclose_pipes() else {
-                perror("pipe");
                 exit_without_destructors(1);
             };
             make_fd_nonblocking(pipes.read.as_raw_fd()).unwrap();

src/fds.rs

@@ -149,7 +149,7 @@ pub fn make_autoclose_pipes() -> nix::Result<AutoClosePipes> {
         Ok(pipes) => pipes,
         Err(err) => {
             FLOG!(warning, PIPE_ERROR);
-            perror("pipe2");
+            perror("pipe");
             return Err(err);
         }
     };

src/io.rs

@@ -345,12 +345,12 @@ pub struct IoBufferfill {
 }
 impl IoBufferfill {
     /// Create an io_bufferfill_t which, when written from, fills a buffer with the contents.
-    /// \returns nullptr on failure, e.g. too many open fds.
+    /// \returns an error on failure, e.g. too many open fds.
     pub fn create() -> io::Result<Arc<IoBufferfill>> {
         Self::create_opts(0, STDOUT_FILENO)
     }
     /// Create an io_bufferfill_t which, when written from, fills a buffer with the contents.
-    /// \returns nullptr on failure, e.g. too many open fds.
+    /// \returns an error on failure, e.g. too many open fds.
     ///
     /// \param target the fd which this will be dup2'd to - typically stdout.
     pub fn create_opts(buffer_limit: usize, target: RawFd) -> io::Result<Arc<IoBufferfill>> {