Improve error reporting when workspace not registered

When the user ran

 tempest run --workspace <whatever>

and the workspace wasn't in .tempest/workspace.yaml the eror message
reported was

 CRITICAL tempest [-] TypeError: coercing to Unicode: need string or buffer, NoneType found.

Rather than report this error, a message is printed saying that the
workspace isn't registered.

Change-Id: Ic539ee2fab83401904fcaec2c3fbf1cd41e4a6e3
3 files changed