Do not rely on backends setting error codes on success

Do not rely on backends setting error codes on success

From the API documentation of KyotoCabinet, TokyoCabinet and QDBM, I am
not convinced that the error code routines consistently return 0 whenever
the previous operation succeeded. Also, I am not convinced that they
return 1 whenever the previous operation failed.
It is important that we do not return 0 on failure, so let's try to get
the error code and fallback to -1 if we couldn't get anything more
specific.

Issue #696