Changes

testenv: less cluttered output for failed cmds

The testenv.cmd.run() already prints the command to run at the start,
prints the command output and blocks until the command is done.

If the command fails, do not print the command again, and do not print a
python stack trace. The reason for the failure is most likely in the
output shown by the program itself (e.g. a prepare= script that failed).
Make it easier for the user to spot this error by uncluttering the
output.

Change-Id: I508322b017e5ed9c21396384ef759a4c49c8d3da