Presenting usable error messages to the user

What do you do to present usable/understandable error messages to the user?

If, for instance, a user tries to upload an image and it fails, I don’t want to give them a “Error! Internal Server Error” message because this will just frustrate them.

dmx-on:error="notifies1.danger('Error! '+lastError.message)"

I want to give them a clearer message that will make them understand the issue and will give them the right wording to pass onto the developer if it is a system issue.

Do any of you have a good solution?