new_variant: return error code if build fails

If the build (launched by new_variant.py) fails, return an error
code after cleaning up as much as possible.

BUG=b:167305316
TEST=See testdata/README.md for full details.
Use `&&` to run a second command if the first one passes.
`cd testdata ; ./new_variant_fulltest.sh hatch && echo PASS`
`cd testdata ; ./new_variant_fulltest.sh waddledoo && echo PASS`
The case with hatch will show "PASS" but because waddledoo has an
issue right now, that line won't show the "PASS".

Change-Id: I6cceeff7127ba93e0bd99d186d3cb1dfca44fcf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2401953
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Jacob Rasmussen <jacobraz@google.com>
Reviewed-by: Greg Edelston <gredelston@google.com>
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
1 file changed