rust-dirs: handle forgotten `Result`s
In 1fe2e574616e I introduced a temporary bugfix to align Rust code with a new
behavior from C/Python and forgot about a few `Result`s (cargo's compiler cache
does not re-emit warnings on cached modules). This fixes it.
For the record, I am still unsure that this behavior change is a good idea.
Note: I was already quite unhappy with the setters and getters for the
`DirstateMap` and, indirectly, `Dirs`, and this only further reinforces my
feelings. I hope we can one day fix that situation at the type level; Georges
Racinet and I were just talking about devising a POC for using the builder
pattern in the context of FFI with Python, we'll see what comes out of it.
Differential Revision: https://phab.mercurial-scm.org/D7609