If build command fails raise a helpful exception (#31)

Author: phawk

lib/install/install.rb

@@ -46,7 +46,7 @@
   say "Add default Procfile.dev"
   copy_file "#{__dir__}/Procfile.dev", "Procfile.dev"
 
-  say "Ensure foreman is install"
+  say "Ensure foreman is installed"
   run "gem install foreman"
 end
 

lib/tasks/cssbundling/build.rake

@@ -1,7 +1,9 @@
 namespace :css do
   desc "Build your CSS bundle"
   task :build do
-    system "yarn install && yarn build:css"
+    unless system "yarn install && yarn build:css"
+      raise "cssbundling-rails: Command css:build failed, ensure yarn is installed and `yarn build:css` runs without errors"
+    end
   end
 end