451580 More specific error message for missing application

- Document that application definition is not automatically included in
  the test runtime.
- If the user configures a specific application for a UI test and this
  application is unknown in the test runtime, provide an error message
  giving the user a chance to figure out the root cause.

Bug: 451580
Change-Id: Ia5aaf2b2c50ee76a7c98beb6cd442551560d71d8