Make ForemanTask.read deal with non-existent tasks

If method `ForemanTask.read` is called and the requested task does not exist,
raise `robottelo.entities.ReadError`. Formerly, the operation would succeed, and
an empty dict would be returned.

Also, fix a few formatting nitpicks. Remove spurious whitespace and parentheses.
Merge pull request #1214 from Ichimonji10/foreman-tasks

Make ForemanTask.read deal with non-existent tasks