Alerting: fix preserving errors in the alert rule state during error to error transitions
Alert state transition from one error to another did not update state.Error correctly.
The error in state.Error remained as the initial error encountered.
This led to another issue, where after a Grafana restart, the error was lost because
the state of the alert rule did not change, but the Error is not preserved in the database
between restarts.
This could happen if the expression service returned an error or the alert routine panicked
during querying.
Alexander Akhmetov
2035814059