Don't attempt to cache sessions after exception

lib/cwmp.coffee

@@ -37,6 +37,7 @@ MAX_CYCLES = 4
 
 throwError = (err, httpResponse) ->
   if httpResponse
+    delete currentSessions.delete(httpResponse.connection)
     httpResponse.writeHead(500, {'Connection' : 'close'})
     httpResponse.end("#{err.name}: #{err.message}")