Bug 1128987 - prevent rhc from needlessly regenerating tokens with x509 ...

...auth
I don't think OptionParser will allow a non-parseable file to be passed in if I'm understanding https://github.com/brenton/rhc/blob/rework_token_fallback/lib/rhc/helpers.rb#L156 correctly.  The exception is caught and the error message is shown to the user.