Throw exception for disabled plugin tasks. Fixes BUKKIT-3951

Without this check, any non-null reference to a plugin is considered
'valid' for registering a task in the scheduler. This is obviously
unintentional behavior and has been changed to throw an
IllegalPluginAccessException. It is now consistent with the
SimplePluginManager event registration contract.

This in affect also addresses BUKKIT-3950 for uninitialized plugin
references (ones without a description).