Tweak mbedTLS handshake error message to be more useful

[Calinou]

• Print error code in hexadecimal instead of decimal.
• Point to the online list of mbedTLS error codes so users can figure out what went wrong.

TODO

• Figure out a permanent URL that lists the error codes in question, or import a map of error codes to strings locally and make use of it.
  • List hexadecimal error codes corresponding to each error constant crocs-muni/usable-cert-validation#132

[Faless]

As far as I understand they are still in the process of moving the documentation, but for now this should have most of the SSL error codes:
https://tls.mbed.org/api/ssl_8h.html

[akien-mga]

Closing as this is an old draft that's not progressing. There's still a need to improve these messages, but so far I don't see a good way to do it. We could link to https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/ssl_8h/?highlight=mbedtls_ssl_handshake#_CPPv421mbedtls_ssl_handshakeP19mbedtls_ssl_context, but as seen above they tend to change URLs, and their docs doesn't give the actual integer value for the error constants... so it's a pain to reverse engineer.