This brings us more into line with other langauges, and makes it more
obvious when we are requiring protobuf generated code.

Change-Id: If7b1cc0f03f609a7f43ddafc8509b44207c60910

This involved fixing a few important bugs in the
Ruby implementation -- mostly cases of mixing
upb field types and descriptor types (upb field
types do not distinguish between int/sint/fixed/sfixed
like descriptor types do).

Also added protobuf-specific exceptions so parse
errors can be caught specifically.

Change-Id: Ib49d3db976900b2c6f3455c8b88af52cfb86e036