oeqa: add output to subprocess exceptions

oeqa: add output to subprocess exceptions

Out of the box subprocess.CalledProcessError.__str__() just displays the command
and exit code, which isn't very useful for debugging.

Add a function to oeqa.utils.subprocesstweak to monkey-patch __str__() so that
it can also display the value of stdout and stderr.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>