Add more safety around the context manager
Add more safety around the context manager

====  Tech Notes ====
- Sanity check Message in handle_add_context(), 'word' needs to be a
string before calling into the context manager.
- Sanity check Message in handle_remove_context(), make sure a context
keyword was received before trying to remove.
- Docstrings
- Suppress exceptions while injecting context

MycroftAI/mycroft-core,Dark5ide/mycroft-core,aatchison/mycroft-core,forslund/mycroft-core,linuxipho/mycroft-core,MycroftAI/mycroft-core,aatchison/mycroft-core,forslund/mycroft-core,linuxipho/mycroft-core,Dark5ide/mycroft-core