Refactor Exceptions into Transfer Exceptions and OAI-PMH Exceptions

[caseyamcl]

Exceptions are getting a little messy. There are basically two types:

• OaiPmhException - These occur ONLY if a valid PHP OAI-PMH Error is sent back. They may be accompanied by a HTTP Error code, or may be included in a 200 Response.
• TransferException - These occur for any situation besides when a valid PHP OAI-PMH error is sent, including:
  • standard HTTP errors,
  • malformed responses (mangled or invalid XML),
  • timeouts, etc.

All exceptions should IMPLEMENT (not inherit from) a common interface