Improve TestLib::system_or_bail error reporting

The original coding was not quoting the complete failing command, and it
wasn't printing the reason for the failure either.  Do both.

This is cosmetic only, so no backpatch.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/202106301524.eq5pblzstapj@alvherre.pgsql

Author: alvherre

src/test/perl/TestLib.pm

@@ -375,9 +375,29 @@ sub system_or_bail
 {
 	if (system_log(@_) != 0)
 	{
-		BAIL_OUT("system $_[0] failed");
+		if ($? == -1)
+		{
+			BAIL_OUT(
+				sprintf(
+					"failed to execute command \"%s\": $!", join(" ", @_)));
+		}
+		elsif ($? & 127)
+		{
+			BAIL_OUT(
+				sprintf(
+					"command \"%s\" died with signal %d",
+					join(" ", @_),
+					$? & 127));
+		}
+		else
+		{
+			BAIL_OUT(
+				sprintf(
+					"command \"%s\" exited with value %d",
+					join(" ", @_),
+					$? >> 8));
+		}
 	}
-	return;
 }
 
 =pod