Remove the check for the clone cluster name. (#270)

* Sanity checks for the cluster name, improve tests.

- check that the normal and clone cluster name complies with the valid
  service name. For clone cluster, only do it if clone timestamp is not
  set; with a clone timestamp set, the clone name points to the S3 bucket

 - add tests and improve existing ones, making sure we don't call Error()
   method for an empty error, as well as that we don't miss cases where
   expected error is not empty, but actual call to be tested does not
   return an error.

Code review by @zerg-junior and @Jan-M