[libFuzzer] Error and exit if user supplied fuzzer writeable directories don't exist

Currently, libFuzzer will exit with an error message if a non-existent
corpus directory is provided. However, if a user provides a non-existent
directory for the `artifact_prefix`, `exact_artifact_path`, or
`features_dir`, libFuzzer will continue execution but silently fail to
write artifacts/features.

To improve the user experience, this PR adds validation for the existence of
all user supplied directories before executing the main fuzzing loop. If they
don't exist, libFuzzer will exit with an error message.

Patch By: dgg5503

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D84808