Add error message context for when saucelabs capactiy limit is reached

this PR explains details of this release:
#285

A developer can be having their tests run in saucelabs,
and alot of the time these tests can fail because of lack of saucelabs capacity.

Currently, when a saucelabs capacity error happens, the error message produced says the following:

13:38:19 Error retrieving a new session from the selenium server
13:38:19 Connection refused! Is selenium server started?

this release just adds a little context to the existing error message, letting the user know the error may have been caused by a saucelabs capacity issue.

Cheers!