1070585: GUI no longer locks on connection test. Adds cancel button.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1070585

The RestlibException.code is the https status code from the error. Would the full Exception _str_ make more sense here? Not sure it matters much since it's just in the log.

Should these be log.warn() or log.info() ?