Improve handling of errors

[jsha]

There were still three places where we could have undefined behavior if C gave us an invalid rustls_result:

• rustls_result_is_cert_error took a rustls_result as a parameter.
• The callback for server certificate validation returned rustls_result.
• The callbacks for session storage returned rustls_result.

Change all of these to use u32 and the try_from implementation.

Fix the comment for the session storage callbacks, which indicated the wrong error strategy.

Remove result_to_error in favor of directly implementing Display on rustls_result. This fixes an issue where CertInvalidData was turned into Either::String rather than Either::Error, which caused rustls_result_is_cert_error to (incorrectly) say it wasn't a cert error. This PR also removes the Either struct and the From impls on it.

Fixes #226

[kevinburke]

Looks OK to me, should we update the changelog now vs. later?

[jsha]

We should update the CHANGELOG as part of the PR - I'll push a commit with that later on.

[djc]

Seems okay to me.