Handle LDAP missing credentials error with a flash message.

If a user fails to provide a username or password to the LDAP login
form then a 500 error is returned due to an exception being raised
in omniauth-ldap. This gem has been amended to use the omniauth
error propagation function (fail!) to pass this exception message to
the registered omniauth failure handler so that the Rails application
can handle it approriately.

The failure function now knows about standard exceptions and no longer
requires a specific check for the OmniAuth::Error exception added by
commit f322975c.

This resolves issue #1077.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>