Not only expose the error message, but also the result message from
calling SSH which contains details about the cause. At the example of
using the wrong SSH key for authentication, with this the console shows
download check: exit status 255: Permission denied (publickey).
instead of just
download check: exit status 255
and the log shows
trace git-lfs: ssh: download with url failed,
error: exit status 255, message: Permission denied (publickey).
instead of just
trace git-lfs: ssh: download attempted with url.
Error: exit status 255
