This pull request partially reverts changes from e16c3b4a44
Original motivation for this assert was described with "A WorkProduct without a saved file is useless"
which was true at the time but now it is possible to have work products with other types of files
(llvm-ir, asm, etc) and there are bugreports for this failure:
For example: https://github.com/rust-lang/rust/issues/123695
Fixes https://github.com/rust-lang/rust/issues/123234
Now existing `assert` and `.unwrap_or_else` are unified into a single
check that emits slightly more user friendly error message if an object
files was meant to be produced but it's missing
