Separate expected errors from unexpected ones in Grafana
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
Separate expected errors from unexpected ones in Grafana: 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