firmware_builder: use check=True with subprocesses
firmware_builder: use check=True with subprocesses

A little easier to keep this clean (and more pythonic) with exceptions
instead of returning error codes.

BUG=b:175248887
BRANCH=none
TEST=./firmware_builder.py build && ./firmware_builder.py test

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: I3764aa5bf3125810e2cacbfc97d9b9d99f6b5aef
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2595809
Commit-Queue: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>
Reviewed-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec