Don't crash sancov if file is unreadable.

Summary:
Caller can provides the list of .so files where some files are
unreadable (e.g linux-vdso.so.1). It's more convenient to handler this in
sancov with warning then making all callers to check files.

Reviewers: aizatsky

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18103

llvm-svn: 263307