don't exec in js:build rake task

this is so evil

test plan:
  * run rake canvas:compile_assets, and verify that the api
    documentation gets built after js:build runs

Change-Id: If931f39735855b13e09ee773f296f24b555b3dc1
Reviewed-on: https://gerrit.instructure.com/9096
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Tested-by: Brian Palmer <brianp@instructure.com>

lib/tasks/js.rake

@@ -47,7 +47,8 @@ namespace :js do
 
   desc "optimize and build js for production"
   task :build do
-    exec("node #{Rails.root}/node_modules/requirejs/bin/r.js -o #{Rails.root}/config/build.js")
+    output = `node #{Rails.root}/node_modules/requirejs/bin/r.js -o #{Rails.root}/config/build.js 2>&1`
+    raise "Error running js:build: \n#{output}\nABORTING" if $?.exitstatus != 0
   end
 
 end