Add rpcexplorer to webapp hierarchy.

web/inc/rpc/rpc-error.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
-  -->
+-->
 
 <script>
   'use strict';
 
   var luci = (function(luci) {
 
     var rpc = luci.rpc = luci.rpc || {};
 
     function GrpcError(code, description) {
       if (code == null) {
         throw Error('code is not set')
       }
       this.name = 'GrpcError';
       this.code = code;
       this.description = description;
       this.message = 'code = ' + code + ' desc = ' + description;
     }
     GrpcError.prototype = Error();
     rpc.GrpcError = GrpcError;
 
     return luci;
   }(luci || {}));
-</script>
+</script>