[llvm-tapi-diff] Replicate diff utility error handling

For scripting purposes, use different error code for cases that aren't a
result of different tbd files, e.g. bad input like a path to a broken
symlink. This behaves more similiarly to how the unix `diff` command behaves.
This also includes minor tweaks on error messages.

Reviewed By: ributzka, JDevlieghere

Differential Revision: https://reviews.llvm.org/D115905