Improve error handling

[mbjorgan]

If a resource exists in the portal and in the code, but not in the statefile the provider errors out with this error message: Error: API returned HTTP 400
Having a lot of resources this can be time consuming to troubleshoot.

It would be nice to have an error message along these lines:
Resource with id/name/whatever already exists - to be managed via Terraform this resource needs to be imported into the State. Please see the resource documentation for "morpheus_resource_xxx" for more information.