run-command: report exec failure

In 321fd823 ("run-command: mark path lookup errors with ENOENT",
2018-10-24), we rewrote the logic to execute a command by looking
in the directories on $PATH; as a side effect, a request to run a
command that is not found on $PATH is noticed even before a child
process is forked to execute it.

We however stopped to report an exec failure in such a case by
mistake.  Add a logic to report the error unless silent-exec-failure
is requested, to match the original code.

Reported-by: John Passaro <john.a.passaro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -728,6 +728,8 @@ fail_pipe:
 	if (prepare_cmd(&argv, cmd) < 0) {
 		failed_errno = errno;
 		cmd->pid = -1;
+		if (!cmd->silent_exec_failure)
+			error_errno("cannot run %s", cmd->argv[0]);
 		goto end_of_spawn;
 	}