ONCALL-718: Differentiate error messaging for public key

[lin-brian-l]

Summary of Changes

This PR adds a different error messages to differentiate between failing to find a public key (when the request to get the key set fails) vs when there isn't a public key matching the JWT. This is to clarify exactly what is causing the error Unable to find public key., which was previously being thrown in two places.

Testing

I copy+pasted my changes to the backend vendor code and verified that, by forcing that exception to be thrown, the error is displayed as expected. I also verified that debugging can be toggled on the service connector by passing true in for the last parameter.

• [ ] I have added automated tests for my changes
• [X] I ran composer test before opening this PR
• [X] I ran composer lint-fix before opening this PR

[lin-brian-l]

I tried Davo's suggestion a second time and it worked, so I've reverted my changes to expose functionality to toggle the debug mode.