Tidy up error raising in magic decorators.

Closes gh-1778

IPython/core/magic.py

@@ -203,7 +203,7 b' def _method_magic_marker(magic_kind):'
                 return decorator(call, func)
             retval = mark
         else:
-            raise ValueError("Decorator can only be called with "
+            raise TypeError("Decorator can only be called with "
                              "string or function")
         return retval
 
@@ -229,7 +229,8 b' def _function_magic_marker(magic_kind):'
             if get_ipython is not None:
                 break
         else:
-            raise('Decorator can only run in context where `get_ipython` exists')
+            raise NameError('Decorator can only run in context where '
+                            '`get_ipython` exists')
 
         ip = get_ipython()
 
@@ -247,7 +248,7 b' def _function_magic_marker(magic_kind):'
                 return decorator(call, func)
             retval = mark
         else:
-            raise ValueError("Decorator can only be called with "
+            raise TypeError("Decorator can only be called with "
                              "string or function")
         return retval