Adding twitter_error_text function

Adding a twitter_error_text function to Net::Twitter::Error.

Returns the error text returned from twitter, without a stack trace 'at line...' bit appended to the end of the string. Modified error() to get the error text from twitter_error_text()

A couple of things I noticed in the pod for error(), didn't want to mess with your docs too much:
-POD for error() mentions that it returns the string '[unknown]' but I don't see this happening in code.
-error() falls back to the HTTP status line, but that isn't mentioned in the POD

Thanks!