FCPPluginMessage: Assert errorCode to be alpha-numeric

- The JavaDoc did require that already, but we didn't check it.
- Use an assert() only for peformance. Change the JavaDoc to say
*should* be alpha-numeric instead of *must* be to ensure that users of
the API don't get surprised by non-alpha-numeric errorCodes since we
only check them with assert().
FCPPluginMessage: JavaDoc

Add missing <br>
FCPPluginMessage.isReplyMessage(): JavaDoc

FCPPluginClient will *not* allow replies to replies to be sent, so we
should say "must not" instead of "should" here.