Show a clearer error for ApplicationPermissionException

If calls to a remote Crowd instance fail with to a permission exception then Crowd wraps the response up as a String in an ApplicationPermissionException. If that response is HTML then the resulting stack trace is likely to be unreadable due to the markup.

RestExecutor.throwError should try to turn the response into something readable. text/plain can be shown directly; text/html should be filtered so only the body text is included in the exception.