Add better error handling to bootDexJar function

aosp/2876754 missed adding error handling to one of the places which
consumes the dex jar deapexed from prebuilt apexes. This CL ensures that
when we have multiple prebuilt apexes _without_ flagging, the hiddenapi
processing code emits a more descriptive error "Multiple deapxers..."
rather than a cryptic error "<module> does not provide a dex jar"

Test: m nothing --no-skip-soong-tests

Change-Id: I80849fa7cca17d724ac53c94bb353b169175ee28