run-command: be more informative about what failed

While debugging an error with verify_signed_buffer() the error
messages from run-command weren't very useful:

 error: cannot create pipe for gpg: Too many open files
 error: could not run gpg.

because they didn't indicate *which* pipe couldn't be created.

Print which pipe failed to be created in the error message so we
can more easily debug similar problems in the future.

For example, the above error now prints:

 error: cannot create standard error pipe for gpg: Too many open files
 error: could not run gpg.
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

run-command.c

@@ -273,6 +273,7 @@ int start_command(struct child_process *cmd)
 	int need_in, need_out, need_err;
 	int fdin[2], fdout[2], fderr[2];
 	int failed_errno = failed_errno;
+	char *str;
 
 	/*
 	 * In case of errors we must keep the promise to close FDs
@@ -285,6 +286,7 @@ int start_command(struct child_process *cmd)
 			failed_errno = errno;
 			if (cmd->out > 0)
 				close(cmd->out);
+			str = "standard input";
 			goto fail_pipe;
 		}
 		cmd->in = fdin[1];
@@ -300,6 +302,7 @@ int start_command(struct child_process *cmd)
 				close_pair(fdin);
 			else if (cmd->in)
 				close(cmd->in);
+			str = "standard output";
 			goto fail_pipe;
 		}
 		cmd->out = fdout[0];
@@ -317,9 +320,10 @@ int start_command(struct child_process *cmd)
 				close_pair(fdout);
 			else if (cmd->out)
 				close(cmd->out);
+			str = "standard error";
 fail_pipe:
-			error("cannot create pipe for %s: %s",
-				cmd->argv[0], strerror(failed_errno));
+			error("cannot create %s pipe for %s: %s",
+				str, cmd->argv[0], strerror(failed_errno));
 			errno = failed_errno;
 			return -1;
 		}