Tighten error handling in slaveRunTaskLabelDecorator hook

For whatever reason, the slaveRunTaskLabelDecorator allows the module author to return an error, but the hook manager "silently" suppresses the error in HookManager::slaveRunTaskLabelDecorator and proceed.

This creates some problems:

1) module author could incorrectly assume that an returned error could cause the task run to fail, but it's actually not the case;

2) module author has not way to instruct Mesos agent to stop the task launch if unrecoverable error happens.

I suggest we tighten the handling here to fail the task run if module reports an error. A module can still work around soft errors by just returning input labels as-is.