mojo: Make clear that sample client dropdown doesn't work.

This also gives a better error message when trying to set the modules to
build, in the event that there is an error.
2 jobs for mojo-err-checking in 1 minute and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #16563
smoke-test

00:01:26

passed #16562
unit-test

00:00:36