[llvm-objcopy] Tidy up error messages

This patch brings various error messages into line with each other, by
removing trailing full stops, and making the first letter lower-case.
This addresses https://bugs.llvm.org/show_bug.cgi?id=40859.

Reviewed by: jhenderson, rupprecht, jakehehrlich

Differential Revision: https://reviews.llvm.org/D62072

Patch by Alex Brachet

llvm-svn: 361384