Separate expected errors from unexpected ones in Grafana

[severo]

We should never have: UnexpectedError, PreviousStepFormatError

Also: some error should be temporary: PreviousStepNotReady, etc.

Instead of one chart with the three kinds of errors, we should have:

• one with the expected errors
• one with the transitory errors
• one with the unexpected errors

By the way, I think we could create intermediary classes (ExpectedError, UnexpectedError, TransitoryError), so that it's clear which error pertains to which category