Creating a new group w/out twitterAPI installed causes fatal error

I tried to create a new group, but when I submitted the form, drupal crashed and I got an unhelpful fatal php error. the form never submitted correctly, so I didn't get to see the drupal message that would've been on the next page saying to install twitterapi correctly.

This patch adds a check for _comms_twitterAPI_username and on failure instead displays a drupal error along with the admonishment to install twitterAPI correctly.