Add message when trying to %load_ext a module that is not an IPython extension.

IPython/core/extensions.py

@@ -87,7 +87,8 b' class ExtensionManager(Configurable):'
         """Load an IPython extension by its module name.
 
         Returns the string "already loaded" if the extension is already loaded,
-        otherwise None.
+        "no load function" if the module doesn't have a load_ipython_extension
+        function, or None if it succeeded.
         """
         if module_str in self.loaded:
             return "already loaded"
@@ -100,6 +101,8 b' class ExtensionManager(Configurable):'
         mod = sys.modules[module_str]
         if self._call_load_ipython_extension(mod):
             self.loaded.add(module_str)
+        else:
+            return "no load function"
 
     def unload_extension(self, module_str):
         """Unload an IPython extension by its module name.

IPython/core/magics/extension.py

@@ -59,9 +59,13 b' class ExtensionMagics(Magics):'
         """Load an IPython extension by its module name."""
         if not module_str:
             raise UsageError('Missing module name.')
-        if self.shell.extension_manager.load_extension(module_str) == 'already loaded':
+        res = self.shell.extension_manager.load_extension(module_str)
+        
+        if res == 'already loaded':
             print "The %s extension is already loaded. To reload it, use:" % module_str
             print "  %reload_ext", module_str
+        elif res == 'no load function':
+            print "The %s module is not an IPython extension." % module_str
 
     @line_magic
     def unload_ext(self, module_str):

IPython/core/tests/test_extension.py

@@ -67,3 +67,7 b' def test_extension_loading():'
             # But can reload it
             with tt.AssertPrints("Running ext2 load"):
                 em.reload_extension('ext2')
+
+def test_non_extension():
+    em = get_ipython().extension_manager
+    nt.assert_equal(em.load_extension('sys'), "no load function")