handle no arguments or non-file/non-existen filenames

When using `pecan serve` we should be able to handle nicely
when no options or bad filenames are given to avoid seeing a
not so helpful traceback.

pecan/commands/serve.py

@@ -1,11 +1,12 @@
 """
 PasteScript serve command for Pecan.
 """
-from paste import httpserver
+from paste              import httpserver
+from paste.script       import command as paste_command
 from paste.script.serve import ServeCommand as _ServeCommand
 
-from base import Command
-
+from base               import Command
+import os
 import re
 
 
@@ -43,10 +44,12 @@ class ServeCommand(_ServeCommand, Command):
         setattr(self.options, 'app_name', None)
         setattr(self.options, 'server', None)
         setattr(self.options, 'server_name', None)
-        
+
+        config_file = self.validate_file(self.args)
+
         # for file-watching to work, we need a filename, not a module
         if self.requires_config_file and self.args:
-            self.config = self.load_configuration(self.args[0])
+            self.config = self.load_configuration(config_file)
             self.args[0] = self.config.__file__
             if self.options.reload is None:
                 self.options.reload = getattr(self.config.app, 'reload', False)
@@ -59,3 +62,8 @@ class ServeCommand(_ServeCommand, Command):
     
     def loadapp(self, app_spec, name, relative_to, **kw):
         return self.load_app(self.config)
+
+    def validate_file(self, argv):
+        if not argv or not os.path.isfile(argv[0]):
+            raise paste_command.BadCommand('This command needs a valid config file.')
+        return argv[0]