Verify the testing database before running tests. (#1174)

To avoid seeing a lot of test failures on the database connection error,
and guide the user how to fix.

Sort the before after blocks in the following actually called order.
See https://relishapp.com/rspec/rspec-core/v/2-99/docs/hooks/before-and-after-hooks

Author: junaruga

spec/spec_helper.rb

@@ -57,12 +57,21 @@ def clock_time
     end
   end
 
-  config.before :each do
-    @client = new_client
-  end
-
-  config.after :each do
-    @clients.each(&:close)
+  config.before(:suite) do
+    begin
+      new_client
+    rescue Mysql2::Error => e
+      username = DatabaseCredentials['root']['username']
+      database = DatabaseCredentials['root']['database']
+      message = %(
+An error occurred while connecting to the testing database server.
+Make sure that the database server is running.
+Make sure that `mysql -u #{username} [options] #{database}` succeeds by the root user config in spec/configuration.yml.
+Make sure that the testing database '#{database}' exists. If it does not exist, create it.
+)
+      warn message
+      raise e
+    end
   end
 
   config.before(:all) do
@@ -126,4 +135,12 @@ def clock_time
       ]
     end
   end
+
+  config.before(:each) do
+    @client = new_client
+  end
+
+  config.after(:each) do
+    @clients.each(&:close)
+  end
 end