We were seeing non-deterministic binary size differences depending on which toolchain was used to build fuchsia. This is because libunwind embeded the FILE path into a logging macro, even for release builds, which makes the code dependent on the build directory.
This removes the file and line number from the error message. This is consistent with how other runtimes report error, e.g. https://github.com/llvm/llvm-project/blob/master/libcxxabi/src/abort_message.cpp#L30.