Add debugging information to error message test (#5239)

One of the test cases in test/error_messages.coffee fails intermittently
in the Node.js ecosystem-testing tool CITGM. In an effort to help debug
what's going on when this occurs, this adds more information to the
AssertionError message in question.

Author: Trott

test/error_messages.coffee

@@ -88,7 +88,7 @@ if require?
 
     eq error.message, 'hello world'
     doesNotThrow(-> error.stack)
-    notEqual error.stack.toString().indexOf(filePath), -1
+    notEqual error.stack.toString().indexOf(filePath), -1, "Expected " + filePath + "in stack trace: " + error.stack.toString()
 
   test "#4418: stack traces for compiled files reference the correct line number", ->
     # The browser is already compiling other anonymous scripts (the tests)