multi_backend.schema doesn't handle error cases of cuttlefish_generator:map/2

Cuttlefish is pretty awesome. One of the awesome things it does is phased error handling. If there's an error in any phase, it returns the 3-tuple `{error, phase_name, [{error, "Message"}]}`

multi_backend is the only translation that recurses back in to recycle the translations and validators from individual backend configurations. If something in there caused a problem, it throws back that awesome error tuple, but doesn't handle it, generating an `[error, function_clause]` instead. this can be fixed by checking for the 3-tuple and passing the error back upstream.