How to copy an Error message from Xcode 8?
I've always just right-clicked onto the warning message and selected Copy. This then places it on the clipboard and is available for pasting into other apps.

Below are two examples of how the copied message looks on the clipboard (or when pasted somewhere else):


  /Users/Username/Documents/Projects/Username (Developer)/iPhone
  Apps/Trivia Apps/Draft Versions/Trivia Apps/App name.xcodeproj Update
  to recommended settings
  
  file:///Users/Username/Documents/Projects/Username%20(Developer)/iPhone%20Apps/Trivia%20Apps/Draft%20Versions/Trivia%20Apps/App%20name.xcodeproj/xcuserdata/Username.xcuserdatad/xcschemes/App%20name.xcscheme:
  warning: Missing file: /Users/Username/Documents/Projects/Username
  (Developer)/iPhone Apps/Trivia Apps/Draft Versions/App
  name.xcodeproj/xcuserdata/Username.xcuserdatad/xcschemes/App
  name.xcscheme is missing from working copy


NOTE: Both of the above were copied from warning messages generated in Xcode 8.2.1, although I have changed the real username and app name.

If I'm misunderstanding your question, please clarify/let me know.