Add hook_services_authentication_info_alter()

Use case: need to customize the output of auth error messages returned by OAuth authentication via REST server.

services_oauth.module registers following auth info:

Array
(
    [file] => services_oauth.inc
    [title] => OAuth authentication
    [description] => An open protocol to allow secure API authorization
    [security_settings] => _services_oauth_security_settings
    [default_security_settings] => _services_oauth_default_security_settings
    [authenticate_call] => _services_oauth_authenticate_call
    [controller_settings] => _services_oauth_controller_settings
)

The, hook_services_authentication_info_alter() allows to easily replace callbacks, following example alters "authenticate_call":

function custom_services_authentication_info_alter(&$info, $module) {
  $info['authenticate_call'] = '_custom_services_oauth_authenticate_call';
}

Patch attached.