Issue #60: alt gexiv2_metadata_try_(g|s)et_tag_multiple() with GError.

Raising GLib warning should not be done on tag errors because calling applications may prefer handling these errors differently (and adding a log handler for every dependency of a software is not flexible). It is much better design to return errors as optional GError.

This is meant to fix #60 (closed). This is very annoying when we get these WARNINGs in GIMP. We prefer to get a GError and handle it appropriately in our own way. Thanks!