Improve parse errors

[segiddins]

Given the following source:

func main() {
  let f: () -> Void = {
    return
  }
  println(f)
}

Turn the error from

error: unexpected token 'return'
 --> test.tr:2:24
  |
 2|   let f: () -> Void = {
  |                        ^

into

error: unexpected token 'return'
 --> test.tr:3:5
  |
  |  let f: () -> Void = {
 3|     return
  |     ^~~~~~ 
  |  }

[harlanhaskins]

Wait, that snippet should compile as-is...

[harlanhaskins]

LGTM! :shipit: