Crash On Unknown Directive

[tomspilman]

If you pass a file with any unknown directive like:

#notgood

You get a crash processing a token:

System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at CppNet.Preprocessor._token()
   at CppNet.Preprocessor.token()

This probably shouldn't crash... it should probably return some unknown directive type. At worst it should output a reasonable error message and fail to parse.