Make the build fail when the executable prebuilt target check fails.

Amends https://r.android.com/1439191; I realised echo-error doesn't
imply a false status.

Test: m art-check-{release,debug,testing}-apex-gen-fakebin
  without https://r.android.com/1441933.
Bug: 169375644
Change-Id: Ice75aeab30120e781df50a28c3dce3874ec0bfd1