Messages of the _error method family inform the sender of a problem on the sender's side.
Standard derivate families of _error are:
- _error_duplicate – something has been provided twice or more.
- _error_illegal – something is not legal.
- _error_invalid – something is not valid.
- _error_necessary – something has not been provided.
- _error_unavailable – something is not available.
- _error_unknown – something does not exist or isn't known.