refactor(falcon.exceptions): Rename module to falcon.errors

It is (arguably) more Pythonic to talk about "errors" as opposed to
"exceptions". Rename the exceptions module to reflect this.

Closes #342
Merge pull request #361 from kgriffs/issues/342

refactor(falcon.exceptions): Rename module to falcon.errors
test(req.uri): Test HTTPS URIs

Add tests to cover HTTPS URIs using default and non-default ports.