Error messages and usability

Error messages are an unavoidable part of the user interfaces. Something can always go wrong and sometimes the user must be notified about it. However a good error message should be both informative and actionable. The user should understand in the user's own language what went wrong and should the user do something about. Writing error message texts is difficult and there are often bad messages that have to be fixed. This opens up an interesting triage of the error message errors.

If the error message can be eliminated altogether, it is a MUST. 
If the error message can teach the user how to fix problem, it is a GOOD TO HAVE. 
If the error message can only give a bit better information, it is a NICE TO HAVE.