[FEATURE REQUEST] Clarify error dialog

If you have more than one UE4 running (it happens) you will get a Can’t save due to an error.

  1. Please consider including text in the error dialog that it’s likely another copy is running and people should close that.
    if there is an OS file error (out of space, locked file, etc) consider including that info so the user can do something constructive about the error.
  2. Please consider a warning if another copy is started on one machine