Improve error message when type cannot be resolved

[nilehmann]

Consider the following

#[flux::opaque]
#[flux::refined_by(b : bitvec <8>)]
pub struct BV8(u8);

impl BitAnd for BV8 {
    type Output = BV8;
    
    #[flux::trusted]
    #[flux::sig(fn(BV8 [@b1], BV8 [@b2]) -> bv_and(b1, b2))]
    fn bitand(self, rhs: BV8) -> BV8 { BV8(self.0 & rhs.0) }
}

It currently fails with

error[E0999]: cannot resolve `bv_and`
  --> attic/playground.rs:11:45
   |
11 |     #[flux::sig(fn(BV8 [@b1], BV8 [@b2]) -> bv_and(b1, b2))]
   |                                             ^^^^^^^^^^^^^^
   |
   = help: flux can only resolve a path if it is present in the definition being refined

bv_and is parsed as a type, but the error doesn't mention type anywhere. The help: is also not true since we implemented the global resolver so it should be removed.

We should give an error similar to what rustc gives

error[E0999]: cannot resolve type `bv_and` in this scope
  --> attic/playground.rs:11:45
   |
11 |     #[flux::sig(fn(BV8 [@b1], BV8 [@b2]) -> bv_and(b1, b2))]
   |                                             ^^^^^^^^^^^^^^
   |                                             not found in this scope