classes/populate_sdk_ext: drop duplicated error message

classes/populate_sdk_ext: drop duplicated error message

The preparation script itself prints out an error on failure, and we
aren't redirecting its output anymore, so we no longer need to print out
a message here when it fails. At the same time, make the message printed
out by the script a little clearer - we're just writing the log out to
the file, we shouldn't give the user an expectation that there will be
extra details in there (other than the output produced by
oe-init-build-env there won't be).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>