(PE-3657) Addressed more PR feedback

This commit includes miscellaneous changes based on PR feedback:

* Cleanup in the reloaded repl example code in the README.md.
  `alter-var-root` -> `check-for-errors!` threaded call replaced with
  nested call.

* Removed overload of `build-app*` which allowed the shutdown promise to
  be omitted from the call, with a default promise being generated
  inside of the function.  Changed `build-app` to create and pass in its
  own promise into `build-app*` to compensate for the removed overload.

* Changed name of `app->shutdown-error-throwable-or-app` to
 `throw-app-error-if-exists!`.

* Removed overloads of `bootstrap-*` functions with support for
  boolean that controlled whether the functions would return an app or
  throw a `Throwable` based on the existence of a shutdown error.
  Functions now always throw a `Throwable` on presence of a shutdown
  error.  Tests needing the "return app on error" behavior were
  updated to call `boot-services-with-config` in Trapperkeeper core
  to achieve the "return app on error" behavior.

* Some unused references to the `:ShutdownService` were removed from
  tests.