Avoid spewing giant debug backtrace into exception in certain OStatus subscription...

Avoid spewing giant debug backtrace into exception in certain OStatus subscription failure cases.

The code pattern 'new XXXException($e)' to chain exceptions doesn't actually work as intended, as exceptions are actually expecting a string message here.
This caused an implicit string conversion from HTTP_Request2_Exception, which is a PEAR_Exception, which defines an absurdly detailed __toString() method including a giant HTML table with a backtrace if you happen to be on a web request.
Simply passing $e->getMessage() instead clears this up, as we'll get the nice short message like 'Couldn't connect to tcp://blahblah:80'