Added help message for gem i webrick in rdoc

lib/rdoc/ri/driver.rb

@@ -1551,7 +1551,12 @@ or the PAGER environment variable.
   # Starts a WEBrick server for ri.
 
   def start_server
-    require 'webrick'
+    begin
+      require 'webrick'
+    rescue LoadError
+      puts "webrick is not found. You may need to `gem install webrick` to install webrick."
+      exit
+    end
 
     server = WEBrick::HTTPServer.new :Port => @server
 

lib/rdoc/servlet.rb

@@ -3,7 +3,13 @@ require 'rdoc'
 require 'erb'
 require 'time'
 require 'json'
-require 'webrick'
+
+begin
+  require 'webrick'
+rescue LoadError
+  puts "webrick is not found. You may need to `gem install webrick` to install webrick."
+  exit
+end
 
 ##
 # This is a WEBrick servlet that allows you to browse ri documentation.