Calling unsupported operations of HDF5 VOL db plugin causes crash

[t-erxleben]

For example here:
https://github.com/julea-io/julea/blob/68dbd67c53c459af437495f0a9988b567ad5f8a0/lib/hdf5-db/jhdf5-db-file.c#L399

When building in debug mode the whole program will crash when this line is reached. It may be more convenient to remove those assertions and return an error value to HDF.