Auto merge of #6506 - mt2d2:darwin_notification_imports, r=metajack

notify_darwin should trap import errors.

notify_darwin should trap import errors and should provide the useful package name for the user to install. Fix for issue #6479.

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python/servo/build_commands.py

@@ -64,6 +64,7 @@ def notify_win(title, text):
 
 
 def notify_darwin(title, text):
+    try:
         import Foundation
         import objc
 
@@ -79,6 +80,8 @@ def notify_darwin(title, text):
 
         centre = NSUserNotificationCenter.defaultUserNotificationCenter()
         centre.scheduleNotification_(note)
+    except ImportError:
+        raise Exception("Please make sure that the Python pyobjc module is installed!")
 
 
 def notify_build_done(elapsed):