Fail ember:compile if build subprocess fails

Closes [#417].

The following is the command `ember-cli-rails` generates for `rake
ember:compile`. It was generated using [the example application][repo].

```bash
/Users/seanpdoyle/src/ember-cli-rails-heroku-example/frontend/node_modules/.bin/ember build --environment 'development' --output-path '/Users/seanpdoyle/src/ember-cli-rails-heroku-example/tmp/ember-cli/apps/frontend' | /usr/bin/tee -a '/Users/seanpdoyle/src/ember-cli-rails-heroku-example/log/ember-frontend.development.log'

Pipes are streaming, so the pipe to `tee` will open before the first
program finishes writing, potentially ignoring a non-zero exit status.

The solution is to set the [`pipefail`][docs] option for the generated
sub-command.

[repo]: https://github.com/seanpdoyle/ember-cli-rails-heroku-example
[#417]: #417
[docs]: http://www.gnu.org/software/bash/manual/html_node/Pipelines.html

lib/ember_cli/command.rb

@@ -14,7 +14,7 @@ def test
     end
 
     def build(watch: false)
-      "#{ember_build(watch: watch)} | #{tee}"
+      "set -o pipefail; #{ember_build(watch: watch)} | #{tee}"
     end
 
     private

spec/lib/ember_cli/command_spec.rb

@@ -25,6 +25,13 @@
       expect(command.build).to match(%r{\| path/to/tee -a 'path/to/log'})
     end
 
+    it "sets pipefail" do
+      paths = build_paths(tee: "path/to/tee", log: "path/to/log")
+      command = build_command(paths: paths)
+
+      expect(command.build).to match(%r{\Aset -o pipefail;})
+    end
+
     context "when building in production" do
       it "includes the `--environment production` flag" do
         paths = build_paths