sanitizer_common: be more verbose, when symbolizer is not found.

Summary:
I have othen been stuck when I got an ASAN report, but no symbols
are resolved. The reasons might be different, and it always 
requires a bit of detective work to track down.

These more verbose error messages will help the users like me.

Reviewers: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14135

llvm-svn: 251553