Fix error handling in needRules() - #1207

I discovered a little bug while trying to import a Pygments lexer: needRules() calls fetchRules() inside compileOnce. This means that fetchRules() is only called the first time needRules() is called. So if the needRules() is called by Tokenise() the error will only be handled the first time. Any call to Tokenise() after that will cause a panic, as no rules are loaded. To fix that, we need to store the error in the struct.

If you want to test it, you can try this invalid lexer in chromad:

<lexer>
  <config>
    <name>Test</name>
  </config>
  <rules>
  </rules>
</lexer>

Just select the Lexer and start typing.