Check if target process has exited first.
Check if target process has exited first.

This is usually the root cause of other expectations failing, so it is
more natural to go first.

Make error message for check more explicit.

Change-Id: I35c8da84f2db1bd38f55be3e3050ffca3c6fba0f
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto