[llvm-nm] Don't report "no symbols" error for files that contain symbols

Previously we were reporting this error if we were list no symbols
which is not the same thing as the file containing no symbols.

Also, always report the filename when printing errors.

This matches the GNU nm behaviour.

This a followup to https://reviews.llvm.org/D52810

Differential Revision: https://reviews.llvm.org/D72658