clang-import-test: Clean up error output for files that cannot be found

Pass on the filesystem error string `FileManager::getFileRef` in
`clang-import-test`'s `ParseSource` function. Also include "error:" and
a newline in the output. As a side effect, migrate to the `FileEntryRef`
overload of `SourceManager::createFileID`.

No real functionality change here, just slightly better output on error.

Differential Revision: https://reviews.llvm.org/D92971