warn on glob with no matches

IPython/nbconvert/nbconvertapp.py

@@ -234,6 +234,8 b' class NbConvertApp(BaseIPythonApplication):'
             # notebooks without having to type the extension.
             globbed_files = glob.glob(pattern)
             globbed_files.extend(glob.glob(pattern + '.ipynb'))
+            if not globbed_files:
+                self.log.warn("pattern %r matched no files", pattern)
 
             for filename in globbed_files:
                 if not filename in filenames: