Handle exceptions in pipeline steps

[jhrozek]

At the moment, any exception in any pipeline step will just raise a 500 from codegate. This is not always desirable.

Ideas on how we could improve:

• mark steps as critical or non-critical
• if a non-critical step fails, log or even add a chunk to the context to be displayed to the user in the conversation but proceed.

[lukehinds]

@lukehinds plan for roadmap