[LockFileManager] Improve error output by using better error messages

This is currently used by clang to lock access to modules; improve the
error message so that clang can use better output messages from locking
error issues.

rdar://problem/26529101

Differential Review: http://reviews.llvm.org/D20942

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271755 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed