extractor error message is not clear

## Minimized code

```Scala
val (a,b) = (1,2,3)
```

## Output

```scala
1 |val (a,b) = (1,2,3)
  |    ^
  |this case is unreachable since type (Int, Int, Int) is not a subclass of class Tuple2
```
and Scala2 gives:
```scala
constructor cannot be instantiated to expected type;
 found   : (T1, T2)
 required: (Int, Int, Int)
```
## Expectation

