Improve stub resolution error

[celinval]

Failure to resolve a stub source or target name is fairly generic. For the following stub:

#[kani::stub(orig2, self::other_crate2::mock)]

an error would look like:

error: unable to resolve function/method: crate::other_crate2::mock

We should be able to provide more information for the user. For example, was the problem found while trying to resolve other_crate2 or mock? Ideally we should also point out where the error is with span_error. Something like the resolution errors from the compiler:

error: cannot find `other_crate2` in the crate root
   --> src/lib.rs:10:20
   |
10 | #[kani::stub(orig2, self::other_crate2::mock)]
   |                           ^^^^^^^^^^^^ cannot find `other_crate2` in the crate root

[celinval]

I tried to improve the error message to provide more information on why the resolution failed. Unfortunately the span of the Ident that represent the path is the same span as the entire attribute.