When a message box is displayed containing a string that's too long for the UI to display, the message is truncated but the entire message is written to the console. The string "(For the full message, see the editor log file)" indicates when this occurs.
This functionality was recently modified to fix bug: https://issuetracker.unity3d.com/issues/editorutility-dot-displaydialog-does-not-show-the-message-when-the-message-is-too-long.
That is, previously the full message text wasn't being written to the console in some cases and this was fixed in 6000.15f1.