mgmt/mcumgr/lib: Fix loop when image write fails

The img_mgmt_upload was getting into loop with requesting
offset 0 from mcumgr, when requested to re-try upload after
write fails.
Because it is not really possible to recover from write fail,
at least currently, the commit changes code to reset upload
state in a case of write error and return an error code.
Now, when write fails, an error will be returned and upload
process will be stopped and reset; upload re-try will behave
as a new upload has been requested.

Fixes #44219

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>