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.

Notice the difference between an _error and a _failure and a _warning (or several other method families):

An _error always indicates a mistake being made on the receiving side of the error rather on the sending side. As if to say "It's your fault, not mine!"

Examples are _error_unknown_setting or _error_excessive_subscriptions.