x/tools/gopls: track uses with fillreturns adding error returns

[stamblerre]

Fillreturns uses any in-scope variables when they match the type, but this doesn't work well with error returns.
An example of this is found in https://www.youtube.com/watch?v=6r08zGi38Tk around the 11:20 mark. The err return variable has already been used in an earlier return statement, but we still return it again below. It would be nice to check that it had already been used and use nil instead.

[matoous]

There are some edge cases that would need to be solved, e.g. (with a grain of salt):

car, err := getCar()
if errors.Is(err, ErrNotFound) {
    return nil, err
}
car.SetDefaults()
return // What do we suggest here?

In this case, the user might still want to return err instead of nil but the err has been already returned once. E.g. the the variable has already been used in an earlier return statement most likely won't work. A lot of cases would be probably covered by checking for

car, err := getCar()
if err != nil {
    return nil, err
}
car.SetDefaults()
return // We can suggest `nil` for err as the error has been "completely" used

Not sure if that makes sense.

Anyway, I think the changes (if any) need to be made here : https://github.com/golang/tools/blob/master/internal/lsp/analysis/fillreturns/fillreturns.go