Add APIError to replace "database query error"

[zanieb]

Summary

To improve error handling in Server and Cloud, we replace the obscure ValueError("database query error") with a simple APIError with a fixed message. This also consolidates the ServerException base into a PrefectBackendException in preparation for shared custom exceptions across Server/Cloud.

Supersedes https://github.com/PrefectHQ/cloud/pull/3103 and will require a follow-up in Cloud (https://github.com/PrefectHQ/cloud/pull/3144) to use these exceptions instead of doubly defined CloudException types.

Importance

Improves exception handling traces at a low level.

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)