Mark error enums as non-exhaustive

[PhilippGackstatter]

Description of change

Mark error enums and other enums to which it is applicable as non-exhaustive. This is in effort to be more future-proof and be able to add more variants in a non-breaking way.

Also feature-gate the encode_jwk function behind flags under which it is actually necessary.

Links to any relevant issues

n/a

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

cargo clippy for the non-exhaustive additions.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes