Add remote error handling

[ZanyMonk]

This PR wraps the entire PHP payload in a try/catch so 500 errors do not appear in the target's log.

• [x] Update channel base class
  • [x] Wrap final payload inside a try/catch
  • [x] Format exceptions and stacktrace
  • [x] Extract eventual exceptions from the response
• [x] Add a catch_errors parameter to all vectors (default: true)
• [x] Add unit tests