1. Fail gracefully if the file fails to decode when loaded.

2. If the load fails, close the half-built edit window.
3. Don't reopen the file to check the shebang.
4. Clarify that we're setting tabs in Tk.

M    EditorWindow.py
M    FileList.py

Lib/idlelib/FileList.py

@@ -33,7 +33,12 @@ class FileList:
             # Don't create window, perform 'action', e.g. open in same window
             return action(filename)
         else:
-            return self.EditorWindow(self, filename, key)
+            edit = self.EditorWindow(self, filename, key)
+            if edit.good_load:
+                return edit
+            else:
+                edit._close()
+                return None
 
     def gotofileline(self, filename, lineno=None):
         edit = self.open(filename)