Send errors to user via chat

[nandanrao]

For survey makers, it is very annoying when there is an error that causes the chatbot to not send any response.

The errors are captured nicely in the State object, so easy to debug for someone who has access to the database, but not easy for a survey programmer.

Examples:

1. A field doesn't exist in metadata
   ...

Ideally, the chatbot can send an error to people when it happens, which will help the survey programmer.