ATets: Print out TF exit code reason

In previous design, ATest will only provide TF's exit code which is not
meaningful to most of the user. This CL makes the exit code display with
an expression.

Bug: 209711525
Test: atest acloud_test (without any device connected)

Change-Id: Icea982768fe09a2754920651880d8ca4b0f02bcb
1 file changed