devtool/sdk.py: error out in case of downloading file failure
It's possible that downloading file from updateserver fails. In
this case, we should error out instead of continue.
We have users reporting unexpected behavior of 'devtool sdk-update'.
When an invalid url is supplied, e.g., `devtool sdk-update http://invalid',
the program reports 'Note: Already up-to-date'.
This is obviously not expected. We should error out in such case.
Signed-off-by: Chen Qi <Qi.Chen@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
