Fail the process if exec fails

When running `install_dependencies` or `compile`, failures won't halt
the host program.

This could lead to false positives and difficult to debug scenarios.

Consumers would prefer this to fail as early on as possible.

lib/ember-cli/app.rb

@@ -339,7 +339,7 @@ def env_hash
 
     def exec(cmd, method: :system)
       Dir.chdir root do
-        Kernel.public_send(method, env_hash, cmd, err: :out)
+        Kernel.public_send(method, env_hash, cmd, err: :out) or fail(cmd)
       end
     end