[fx build] Emulate -k 0 behavior across zircon and fuchsia builds

The `-k 0` switch would instruct the build to keep going in case of
errors. This patch emulates that behavior across two invocations of
ninja, for zircon and fuchsia.

TEST: run builds with errors and verify that the build continues and
also returns an error code

Change-Id: I105e2cfbea61c24a503cd70570d5c3ad129c4cba