Our user backends log exceptions thrown during authentication but they don't throw
a exception to inform the caller that an error occured. The new `AuthenticationException'
class should be thrown in that case.
This exception should be fired when the OS restricts the web user
from executing certain actions. Also the template for errors has
been modified so it displays the action and the target that
was prevented.
refs #4092
Eric Lippmann
Marius Hein
Jannis Moßhammer