Throw error if grammar cannot be resolved for path

src/app/language-mode.coffee

@@ -55,6 +55,7 @@ class LanguageMode
       @grammar = @buffer.project.grammarForFilePath(path, pathContents)
     else
       @grammar = syntax.grammarForFilePath(path, pathContents)
+    throw new Error("No grammar found for path: #{path}") unless @grammar
     previousGrammar isnt @grammar
 
   isQuote: (string) ->