non-transient handlers can stay in a bad state in case of exceptions

Handlers like the QueryHandler can stay in a bad decoding state if an exception occurs during the doEncode call.
Exceptions are caught in AbstractGenericHandler in order to be forwarded to the request's Observable, but it has 2 shortcomings:

1. the state isn't reset (and specific state like queryParsingState isn't reset either)
2. if the parsing is done in a "streaming" fashion where a CouchbaseResponse is sent early, and the exception occurs after that fact, the observable will have been completed and cannot actually emit the error.