cgpt.py: report section name when asserting bad format

It is good to know what exact layout section is in error when the
conflicting setting assertion is raised.

BUG=none
TEST=tried running cgpt.py with an incorrect json file, observed
     section name reported in the error message.

Change-Id: I4ba7d60913414c8ea681c10410aa9d45da67e9ce
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/250833
Reviewed-by: Daniel Ehrenberg <dehrenberg@chromium.org>
1 file changed