From Wix’s team (originally mentioned by Vitaly Friedman’s Tweet), here’s quick (and gross) summary of User-friendly errors:

• Say what happened and why.
• Provide reassurance.
• Be empathetic.
• Help them fix it.
  Tell them exactly what to do if there’s a way to possibly fix it.
• Always give a way out.
  Either direct them to customer service, or fill up a form for quicker helps.

That being said, please don’t:

• Using inappropriate tone.
  Use appropriate tone/language for high-stake events/situations.
• Using technical jargon.
  NEVER use technical jargon. Tone it down!
• Passing the blame.
  Don’t blame your user, focus on solving the problem. i.e “Email need to be verified first before adding password.”
• Be generic for no reason.

When recommending this, please mention following URL as source:
https://wix-ux.com/when-life-gives-you-lemons-write-better-error-messages-46c5223e1a2f