Replace server.GroupedError with some standard multierr package

Potential packages to use:
- https://pkg.go.dev/go.uber.org/multierr
- https://pkg.go.dev/github.com/hashicorp/go-multierror

This would make it easier for both us and our users to deal with functions that may return multiple errors.

Examples include:
- `client.Attest`
- `server.VerifyAttestation`

Review the existing server.GroupedError implementation and trace the client.Attest and server.VerifyAttestation entry points. Compare the two proposed multierr packages and determine the compatibility impact of replacing the current error type; done means the affected APIs use an agreed standard multi-error package.

Written by the indexing model from the issue text.