GUACAMOLE-990: Ensure internal errors during auth reach global error handling/logging.

[mike-jumper]

The recent changes adding support for automatically banning IP addresses that repeatedly fail to authenticate (#758) have resulted in the details of internal errors being omitted from the Tomcat logs, instead being logged as “User authentication was aborted”.

For example, if PostgreSQL is being used but the database server has not been started, the following is logged:

12:12:59.935 [http-nio-8080-exec-2] WARN  o.a.g.e.AuthenticationProviderFacade - The "postgresql" authentication provider has encountered an internal error which will halt the authentication process. If this is unexpected or you are the developer of this authentication provider, you may wish to enable debug-level logging. If this is expected and you wish to ignore such failures in the future, please set "skip-if-unavailable: postgresql" within your guacamole.properties.
12:12:59.949 [http-nio-8080-exec-2] ERROR o.a.g.rest.RESTExceptionMapper - Request could not be processed: User authentication was aborted.

These changes rethrow the underlying cause when authentication fails due to an unchecked internal error. For other errors (subclasses of GuacamoleException), the original underlying exception is unwrapped and rethrown as before.