Improve error message when Puma isn't available

UPGRADING.md

@@ -1,5 +1,13 @@
 # Upgrading from Capybara 2.x to 3.x
 
+## Server
+
+Capybara 3 changes the default server to <a href="https://github.com/puma/puma">Puma</a>. If you do not use `Puma` in your project you can
+revert to the previous default of WEBRick by specifying
+
+```ruby
+Capybara.server = :webrick
+```
 
 ## Finders
 

lib/capybara.rb

@@ -438,7 +438,11 @@ Capybara.register_server :webrick do |app, port, host, **options|
 end
 
 Capybara.register_server :puma do |app, port, host, **options|
-  require 'rack/handler/puma'
+  begin
+    require 'rack/handler/puma'
+  rescue LoadError
+    raise LoadError, "Capybara is unable to load `puma` for its server, please add `puma` to your project or specify a different server via something like `Capybara.server = :webrick`."
+  end
   # If we just run the Puma Rack handler it installs signal handlers which prevent us from being able to interrupt tests.
   # Therefore construct and run the Server instance ourselves.
   # Rack::Handler::Puma.run(app, { Host: host, Port: port, Threads: "0:4", workers: 0, daemon: false }.merge(options))