SecretStore: merge two types of errors into single one + Error::is_non_fatal (#8357)

* SecretStore: error unify initial commit

SecretStore: pass real error in error messages

SecretStore: is_internal_error -> Error::is_non_fatal

warnings

SecretStore: ConsensusTemporaryUnreachable

fix after merge

removed comments

removed comments

SecretStore: updated HTTP error responses

SecretStore: more ConsensusTemporaryUnreachable tests

fix after rebase

* fixed grumbles

* use HashSet in tests