Backpack silently substitutes missing signatures with empty ones instead of throwing an error

Motivation

It's easy to mistakenly name signature files with .sig instead of .hsig. In such a situation, instead of throwing an error, backpack will assume a file with no definitions and proceed to throw an error during renaming instead of on the cabal-level. With how buggy that part of the codebase can sometimes be, it makes it often difficult to figure out whether the error stems from backpack or the code compiled with it.

Proposal

Throw an error when a signature file is not found instead of silently assuming an empty signature