Uniform validation errors type

[pracucci]

Following up the discussion in #3167 (review), we should uniform the error type used for validation errors.

[bboreham]

I would like to see an error type that neatly expresses what went wrong without mentioning the text "http" or "grpc", neither of which are relevant.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid, but low priority

[bboreham]

Still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.