Provide more specific error handling.

[LATaylor-guardian]

Example: Cancel reindex

There are three error outcomes here:

1. The request to cancel the reindex from the content source tool fails and therefore this is an internal server error.
2. No content source exists for the given content source and therefore this is a bad request.
   3.There is no running reindex for this content source and therefore this is a bad request.

I think the CustomError type impedes here. Maybe I can group those specific errors under a new type, CancelReindexError and match on that to resolve the error response more specifically.