hw/arm/nseries: Check return value from load_image_targphys()

hw/arm/nseries: Check return value from load_image_targphys()

The nseries machines have a codepath that allows them to load a
secondary bootloader.  This code wasn't checking that the
load_image_targphys() succeeded.  Check the return value and report
the error to the user.

While we're in the vicinity, fix the comment style of the
comment documenting what this image load is doing.

Fixes: Coverity CID 1192904
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201103114918.11807-1-peter.maydell@linaro.org