clang-rename: improve error message when -old-name is used and could not find symbol

Old output was:

clang-rename: could not find symbol at tools/clang/tools/extra/test/clang-rename/ClassFindByName.cpp:1:1 (offset 0).

Reviewers: omtcyfz

Differential Revision: https://reviews.llvm.org/D24002

llvm-svn: 280062