GUACAMOLE-684: Merge changes giving tentative acceptance of credentials priority over complete refusal.
As described in the discussion surrounding the original pull request:
* An extension throws `GuacamoleInsufficientCredentialsException`
specifically to indicate tentative acceptance of the credentials
passed thus far.
* Just as such an extension that fully accepts credentials takes
priority over an extension that refuses to accept the same, it makes
sense to allow an extension that *tentatively* accepts those credentials
to also take priority.
With the above perspective, authentication result priorities are as
follows, with ties broken by the inherent order of the auth providers:
1. Acceptance (returning an `AuthenticedUser` instance).
2. Tentative acceptance (throwing
`GuacamoleInvalidCredentialsException`).
3. Complete refusal (any other subclass of
`GuacamoleCredentialsException`).
4. Neither refusal nor acceptance (returning `null`).
See: https://github.com/apache/guacamole-client/pull/352