Avoid hiddenapi ignoring prebuilt with missing dex implementation jar

Previously, when a prebuilt was preferred but did not provide a
suitable boot dex jar both the source and the prebuilt were silently
ignored which meant that the "hiddenapi list" command was not given a
complete set of boot dex jars. That could either lead to incorrect
hiddenapi flags being set or the "hiddenapi list" command failing if it
could not find a class. Debugging the cause of either of those cases
can be very time consuming so this change fails early and makes the
cause very explicit.

Bug: 181267622
Test: m nothing
Change-Id: I6763ddb9ba90ed2e501d0cf7984f6655237e905d