Improve/Cleanup ObjC NSError reporting

[thomasvl]

• GPBExceptionMessageKey is left in the code, but isn't used, it can go.
• parse*FromData:…/initWithData: methods currently only return two error codes, see if we can do better. Under the hood, the CodedInputStream will raise a few different errors based on what fails. We likely can standardize those so we can expose explicit NSError codes for things like:
  • Hitting the recursion limit
  • malformed data (bad tags)
  • truncated data (needed more bytes then are available)

[thomasvl]

#1620 removed GPBExceptionMessageKey