int64_zigzag/2, uint32_codes, etc fail/error in a confusing way

[kamahen]

This applies to all the predicates defined in protobufs.c

If an argument is instantiated and not of the appropriate type (integer, list(integer), etc., sometimes the predicate quietly fails, sometimes gives an incorrect "instantiation error", sometimes gives an appropriate error.

Implementation notes:
See PL_term_type() and return values PL_VARIABLE, PL_INTEGER.

Need to verify all instantiation errors and also add checks before some unification calls.

[kamahen]

float32_codes/2 and float64_codes/4 should be modified to be similar to the other XXX_codes/2 predicates.

[kamahen]

This is mostly fixed in #10