Incorrect error locations for some syntax errors which refmt gets right

Tested with esy and Reason 3.4.0.

- [ ] Wrong arrow in pattern matching case (`->` instead of `=>`). `refmt` gets the error location correct.
```reason
let f = x =>
switch x {
| () -> /* Error should be at this arrow*/ ()
}; /* Error shows up at the end of the file */
```

- [ ] Incomplete function definition. `refmt` says there's an error at the start of line 2, which is reasonable. vim-reasonml marks the `l` in `let` and simply has the message `invalidCharacter.orComment.orString`
```reason
let f = x =>
```

- [ ] `let` where it doesn't belong. Same error/issue as the previous example.
```reason
let f = () =>
let
```

Reproduce the three Reason syntax-error examples in vim-reasonml and compare their reported locations with refmt, using the cases in the issue as the starting point. Trace the plugin's error-location handling for these inputs; done means each location matches the behavior described as correct for refmt.

Written by the indexing model from the issue text.