After many tangents (also see https://github.com/sourcegraph/sourcegraph/pull/31431) I decided to rip off the bandaid - this PR is a wholesale migration away from go-multierror into a custom multierror implementation that is fully compatible with cockroachdb/errors, prints all errors, can be introspected with Is, As, and friends, and more. The new multi error type is only available as an interface.
resolves our core issue of multi errors being swallowed when wrapped, potentially omitting critical debugging information from logs
nicer way to get multi errors: errors.As into the MultiError interface, then use the read-only Error(). No more initializing and passing around the multi error struct
removes the need for ErrorOrNil! All multi-errors can be treated transparently as an error with no special handling - you can always rely on err != nil to be trustworthy
turns CombineErrors into multi errors as well, because the behaviour of it appears quite deceptive.
It also gets rid of random ways errors are joined into multi errors:
no more appending directly to err.Errors
no more magic _ = Append(multi, err) - we don't modify the provided errors anymore
This PR also removes a few error list formatters into custom error types, which is probably what they should have been anyway.
