Fix noisy console messages about unported add-ons by default.

* Make those single-line warning, there is really no reason to raise
exception (i.e. error) for that.
* Only show them when `--debug` option is set.

release/scripts/modules/addon_utils.py

@@ -365,11 +365,9 @@ def enable(module_name, *, default_set=False, persistent=False, handle_error=Non
         # 1.1) fail when add-on is too old
         # This is a temporary 2.8x migration check, so we can manage addons that are supported.
 
-        try:
-            if mod.bl_info.get("blender", (0, 0, 0)) < (2, 80, 0):
-                raise Exception(f"Add-on '{module_name:s}' has not been upgraded to 2.8, ignoring")
-        except Exception as ex:
-            handle_error(ex)
+        if mod.bl_info.get("blender", (0, 0, 0)) < (2, 80, 0):
+            if _bpy.app.debug:
+                print(f"Warning: Add-on '{module_name:s}' has not been upgraded to 2.8, ignoring")
             return None
 
         # 2) try register collected modules