Better errors from build-flag

When `get_build_var` fails, output the error.

Bug: none
Test: manual
Ignore-AOSP-First: cherry-pick
Merged-In: If4dc13e0e895823f50de1f5c7e929bb05eac69a0
Change-Id: If4dc13e0e895823f50de1f5c7e929bb05eac69a0