Simplify missing deps handling in buildBootImage

Previously, the buildBootImage function maintained a list of
missingDeps that was returned from findAndCopyBootJars and was passed
to various other methods that created ninja rules that depended on the
files created by findAndCopyBootJars. The purpose of that list was
basically to record that findAndCopyBootJars could not find some files
so that the subsequent ninja rules could be marked as being in error.

This changes avoids having to explicitly propagate the missing deps to
each rule by instead having findAndCopyBootJars generate an error rule
instead of a Cp rule for any missing file. That means that any attempt
to build any rule that depends on the missing file will fail when the
missing file is created.

Bug: 177892522
Test: Add an unknown module to PRODUCT_BOOT_JARS
      lunch art_module_arm64
      - Run each of the following targets and made sure it failed with a
        message indicating that the unknown module was missing.
      m SOONG_ALLOW_MISSING_DEPENDENCIES=true out/soong/module_arm64/dex_bootjars/boot.prof
      m SOONG_ALLOW_MISSING_DEPENDENCIES=true out/soong/generic_arm64/dex_bootjars/boot.bprof
      m SOONG_ALLOW_MISSING_DEPENDENCIES=true out/soong/generic_arm64/dex_bootjars/updatable-bcp-packages.txt
Change-Id: Ib7196c128419fbffdfeb61c1c5d47a3b08d1e132