Print the error message the backend produces on HTTP error
Print the error message the backend produces on HTTP error

This is more useful than a backtrace.  I was getting backtraces when the
server was returning the error message:

> No 'remote_control' parameter specified, and
> 'test_pack.default_remote_control' isn't specified in your stbt.conf
> configuration file either.

Now the error message is just printed it's easy to see what went wrong.

stb-tester/stbt-rig