Add error condition for plugin usage message

include/sardine.py

     pass
 
 
+class SardinePluginUsageError(SardinePluginError):
+    pass
+
+
 class XGPRenderer(object):
 
     def __init__(self, txt=None):
 
 class App:
 
+    name = "sardine"
+
     def __init__(self, cmdargs):
         self.cmdargs = cmdargs
         self._set_pluginpath()
         try:
             self.subcommand = Subcommand(self.cmdargs, self.pluginpath)
             print self.subcommand.get_output()
+        except SardinePluginUsageError as e:
+            msg = ("plugin usage: %s %s %s"
+                   % (self.name, self.subcommand.name, e))
+            self.die(msg, 11)
         except SardinePluginError as e:
             self.die(e, 10)