Error message should advice to use 'mysql2' gem.

If I create a new app using command
rails new demo -d mysql
then Gemfile contains 'mysql2' gem by default.

However if mysql gem is missing then error message says
raise "!!! Missing the mysql gem. Add it to your Gemfile: gem 'mysql', '2.8.1'"

[#5569 state:committed]

Signed-off-by: Santiago Pastorino <santiago@wyeworks.com>

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -19,11 +19,11 @@ def self.mysql_connection(config) # :nodoc:
         begin
           require 'mysql'
         rescue LoadError
-          raise "!!! Missing the mysql gem. Add it to your Gemfile: gem 'mysql', '2.8.1'"
+          raise "!!! Missing the mysql gem. Add it to your Gemfile: gem 'mysql2'"
         end
 
         unless defined?(Mysql::Result) && Mysql::Result.method_defined?(:each_hash)
-          raise "!!! Outdated mysql gem. Upgrade to 2.8.1 or later. In your Gemfile: gem 'mysql', '2.8.1'"
+          raise "!!! Outdated mysql gem. Upgrade to 2.8.1 or later. In your Gemfile: gem 'mysql', '2.8.1'. Or use gem 'mysql2'"
         end
       end