Introduction of error codes for extension writing functions

[hanno-becker]

The current 1.3 prototype allows extension writing functions to fail, see e.g. #2. This deviates from upstream Mbed TLS where extension writing functions silently return on an error and must guarantee to not have changed any state in this case.

I think the correct way is to use error codes, but this should be unified with upstream Mbed TLS once we upstream.