Somehow the combined output does not contain the stderr in the errorn
only in the buffer.
For example:
err = "error exit 1"
buf = "Error EBUSY: not enough monitors would be available () after stopping mons [a]"
So we now embed the buf in the error so that the caller does not need to
print the buffer as well as the error.
Also, the caller can more easily decide to ignore the error by
instropecting the string.
Signed-off-by: Sébastien Han <seb@redhat.com>
