Add in callback support for when a notify stage is unsupported

When running bugsnag in an environment which is not specified within releaseStages, no callback is called if one is given - need to call callback as though everything went OK for codebase to continue as expected without needing more if statements around code in codebase.

Took the liberty to move a block of code which returns out of notify so that it's a little more efficient, it doesn't need to be within notifyError, seems as though no reason for it not to go in notify. Correct me if I'm wrong :smile: