payment: handle case when a plugin is not available

Make sure to always throw the right error in case the plugin cannot be
found.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
api: add PaymentApi#refreshPaymentMethods for all plugins

Add a call to refresh all payment methods across all plugins.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Merge branch 'master' of github.com:killbill/killbill