Turn missing jarjar output files into errors

Jarjar doesn't exit with a nonzero return code when there is a syntax
error in a rules file and doesn't write the output file.  Without a
nonzero return code ninja prints a warning but continues, which leads
to stale results on incremental builds where the output file already
exists, or delayed errors on clean builds whre the output file didn't
exist.

Delete the output file before running jarjar, and then check if it
exists after running jarjar.

Fixes: 119516143
Test: m out/target/common/obj/JAVA_LIBRARIES/noto-emoji-compat-java_intermediates/classes-jarjar.jar
Change-Id: Ib99167403948ee11f039dd4d2d7e63052bb92136