Improve svcat's "x is required" error messages & make them consistent

[luksa]

For example, svcat provision clearly specifies what name is required:

# svcat provision
Error: an instance name is required

But others return slightly less-descriptive errors like this:

# svcat deprovision
Error: name is required 

[jboyd01]

I restarted the travis job: The job exceeded the maximum time limit for jobs, and has been terminated.

[carolynvs]

LGTM

Thanks for making the error messages more clear! 👍