Carry x/gov proposal execution error message

[robert-zaremba]

Problem Definition

Today, when a proposal fails the following happens:

• Proposal is removed from a queue
• Proposal status is set to failed
• Event is emitted with "proposal_failed" value.

There is no trace in the standard flow with a reason why the proposal failed. It's only logged in the standard logger.
So the only way to know why a proposal failed, is to grep logs.

Proposal

Attach the error message in a Proposal object or in an Event.

I think the event option makes more sense.