Raise BuildError when a build or install command fails

[tricknotes]

Summary

Runner#run! exited the calling process when its command failed, so a failing ember build or dependency installation terminated any Ruby process running it — callers of EmberCli.compile! and EmberCli.install_dependencies! included. App#compile also runs during request handling in development, where a build failure could take the Rails server down with it.

• Runner#run! now raises EmberCli::BuildError, naming the command and its exit status, instead of calling exit
• The rake tasks (ember:compile / ember:install) still exit nonzero, now through the uncaught exception
• A request-time build failure surfaces the same way BuildMonitor#check! failures already do (both raise BuildError)
• Update the Runner#run! spec (which documented the SystemExit) and add a CHANGELOG entry

This completes the exit removal started for ember test in #660, which raises EmberCli::TestFailureError since 0.13.1.

Verification

• spec/lib: 126 examples, with the only local failure being the known Chrome-dependent App#test example (passes on CI); the App#compile example exercises a real ember build through the new path