lib: Add AuthenticationException for exceptions thrown during authentication

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.

library/Icinga/Exception/AuthenticationException.php

@@ -0,0 +1,14 @@
+<?php
+// {{{ICINGA_LICENSE_HEADER}}}
+// {{{ICINGA_LICENSE_HEADER}}}
+
+namespace Icinga\Exception;
+
+use RuntimeException;
+
+/**
+ * Exception thrown if an error occurs during authentication
+ */
+class AuthenticationException extends RuntimeException
+{
+}