Display notification when branch is not mapped to any app

When current branch isn't mapped to any app, `hk show-build` output looks like this:
```
$ hk show-build
Branch experiment-branch is not mapped to any Heroku app. Point it to a Heroku app with

  git config branch.experiment-branch.heroku <heroku-app-name>

or specify a Heroku app with the -a option.
```
instead of:
```
$ hk show-build
Builds on :
error: The requested API endpoint was not found. Are you using the right HTTP verb (i.e. `GET` vs. `POST`), and did you specify your intended version with the `Accept` header?
```