Addes test for error messages when operating on non-existing files.

tests/test-notfound

@@ -0,0 +1,9 b''
+#!/bin/sh
+
+hg init
+
+echo "Is there an error message when trying to diff non-existing files?"
+hg diff not found
+
+echo "Is there an error message when trying to add non-existing files?"
+hg add not found

tests/test-notfound.out

@@ -0,0 +1,9 b''
++ hg init
+Is there an error message when trying to diff non-existing files?
++ hg diff not found
+not: No such file or directory
+found: No such file or directory
+Is there an error message when trying to add non-existing files?
++ hg add not found
+not: No such file or directory
+found: No such file or directory