Provide better information w/ 500 error in EAS validation

* HTTP error 500 occurs with Exchange 2003 when the user tries to
  authenticate with wrong credentials; we report this as "Cannot
  connect to server", which is wrong
* Error 500 is supposed to mean "internal server error", but since
  we know that an authentication error can cause this, we now put
  up a better error message

Bug: 2933381
Change-Id: I9319a5ed6fbf4c92739f305b42b6b1cad7036a4b
2 files changed