The failure lists were falling out of date because the
tests would pass even if people forgot to remove failures
from the list.

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

This allows us to enable conformance tests even when we know
that some tests are failing and need to be fixed.

Change-Id: I372f43663008747db6f2b2cf06e6ffa4c6d85b2d

Change-Id: I85675bfcd7697bb058b9af475445a6bfdd7fd967

Change-Id: I4c81808e6ace77d2b5737a43417045321b0b10f0

This is necessary for testing on iOS.

Change-Id: I54ec1e3aa2e9fbfff9a6cd6580920a6a62069b63