Proper ACME error handling

[jgnagy]

In the CA service, there are a few places where we raise custom errors that neither use the proper content type nor correspond to ACME error types (e.g., line 187, almost certainly others).

If possible, we should convert these to an actual ACME error type (even a generic one if the spec doesn't specify an appropriate one for the situation) so that clients can handle the issue correctly.

[Om-Beast]

Hi @jgnagy! I'd like to work on this issue.

I'll review the current custom ACME error handling, replace it with the appropriate ACME error types where applicable, and ensure the correct content type is returned. If there are any implementation preferences or edge cases I should be aware of, please let me know.

Could you please assign this issue to me? Thanks!

[jgnagy]

Certainly, @Om-Beast! I look forward to your contribution. Let me know if you have any questions or if you need any help getting started.