Add stderr to unit test error messages.

Messages on stderr were being lost due to the error suppression used to customize the error message.

Also update the formatting to be more informative and concise.

test/lib/pgBackRestTest/Common/ExecuteTest.pm

@@ -233,7 +233,9 @@ sub endRetry
         if ($self->{bSuppressError})
         {
             &log(DEBUG, "suppressed error was ${iExitStatus}");
-            $self->{strErrorLog} = '';
+
+            # Make a copy of the suppressed error log in case it is needed
+            $self->{strSuppressedErrorLog} = $self->{strErrorLog};
         }
         else
         {