examples/deepzoom_server: Add earlier check for slide file

When run from the command line, verify during startup that a slide file
was specified, rather than waiting until the first request.

Author: bgilbert

examples/deepzoom/deepzoom_server.py

@@ -156,6 +156,7 @@ def slugify(text):
     try:
         app.config['DEEPZOOM_SLIDE'] = args[0]
     except IndexError:
-        pass
+        if app.config['DEEPZOOM_SLIDE'] is None:
+            parser.error('No slide file specified')
 
     app.run(host=opts.host, port=opts.port, threaded=True)