Raise a better error message when testing_support/common_rake can't b…

…e found

Fixes solidusio#2041

Rakefile

@@ -2,7 +2,12 @@ require 'rake'
 require 'rubygems/package_task'
 require 'thor/group'
 require File.expand_path('../core/lib/generators/spree/install/install_generator', __FILE__)
-require 'spree/core/testing_support/common_rake'
+begin
+ require 'spree/core/testing_support/common_rake'
+rescue LoadError
+ raise "Could not find spree/core/testing_support/common_rake. You need to run this command using Bundler."
+ exit
+end
 
 spec = eval(File.read('spree.gemspec'))
 Gem::PackageTask.new(spec) do |pkg|