Improve error handling of beautify and testing

spec/beautify-languages-spec.coffee

@@ -184,7 +184,7 @@ describe "BeautifyLanguages", ->
                         beautifyCompleted = false
                         completionFun = (text) ->
                           try
-                            expect(text instanceof Error).not.toEqual(true, text.toString())
+                            expect(text instanceof Error).not.toEqual(true, text.message or text.toString())
                             return beautifyCompleted = true if text instanceof Error
                           #   logger.verbose(expectedTestPath, text) if ext is ".less"
                           #   if text instanceof Error

src/beautifiers/executable.coffee

@@ -179,7 +179,7 @@ class Executable
               if @isWindows() and returnCode is 1 and stderr.indexOf(windowsProgramNotFoundMsg) isnt -1
                 throw @commandNotFoundError(exeName, help)
               else
-                throw new Error(stderr)
+                throw new Error(stderr or stdout)
             else
               if returnStderr
                 stderr