fix: apply custom error handler to KsqlStatementException (MINOR)

[vcrfxia]

Description

The custom error handler in KsqlResource is currently only applied to raw exceptions and KsqlExceptions, but not KsqlStatementExceptions. This means the error handler misses cases such as when Schema Registry is not enabled, because the DefaultSchemaInjector has logic to wrap the KsqlException in a KsqlStatementException.

We could fix this by not having injectors (and other parts of the code) wrap KsqlExceptions, but it seems better to leave that flexibility and instead apply the error handler to KsqlStatementExceptions from KsqlResource. At least, I see no reason not to.

Testing done

Added unit tests.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[pgaref]

LGTM, thanks Victoria!

Just for completeness, seems like these are the 2 checks we are missing by not invoking the custom error handler getting default responses instead of the specific ones