[libFuzzer] Print a correct error message when a directory can't be

opened.

Summary:
Currently if the directory cannot be opened for a reason other than
non-existence (e.g. too many open file descriptors) the error message
printed is incredibly confusing.

Patch By: Alex Gaynor

Reviewers: kcc, morehouse

Reviewed By: morehouse

Subscribers: delcypher, llvm-commits, Sanitizers

Differential Revision: https://reviews.llvm.org/D45322

llvm-svn: 329438