#### Problem

The `Exit` group of error-handling macros inhibit scoping, and
are unnecessary when there is no cleanup at the `exit:` label.

#### Summary of Changes

- Remove `exit: return` idiom from src/lib, replacing `Exit` macros
  with corresponding `Return` macros.
- Incidentally fixed a bug in `RawEndPoint::BindInterface` and
  `UDPEndPoint::BindInterface` that could leave the LwIP lock held
  by calling `SuccessOrExit()` within a locked section.