Be more descriptive in error message.

Carreau · Carreau · commit fa0c8cb13f70 · 2015-08-18T10:22:27.000+02:00
diff --git a/IPython/external/mathjax.py b/IPython/external/mathjax.py
@@ -9,5 +9,5 @@
 import sys 
 
 if __name__ == '__main__' :
-    sys.exit("IPython.external.mathjax is deprecated")
+    sys.exit("IPython.external.mathjax is deprecated, Mathjax is now installed by default with the notebook package")
 
