Omnisci typesystem is a subset of RBC typesystem

[pearu]

@omnisci('uint32(uint32)')
def foo(i):
    return i
omnisci.register()

fails:

                        rtype = ext_arguments_map[sig[0].tostring(
>                           use_annotation=False)]
E                       KeyError: 'uint32'
rbc/omniscidb.py:698: KeyError

Implement better error handling when users try to use types that Omnisci does not support.