[llvm-nm] Tidy up error messages

This adds colons to separate the file name from the message, removes a
duplicate space, and removes a trailing full stop from some messages.
These help bring the error messages into line with other tools, as well
as making all llvm-nm message more self-consistent.

Differential Revision: https://reviews.llvm.org/D96601

Reviewed by: Higuoxing, rupprecht, MaskRay