`catch` block introduces extra var

```coffee
try
catch e
```

Actual:

```js
// Generated by CoffeeScript 2.6.1
var e;

try {

} catch (error) {
e = error;
}
```

Expected:

```js
// Generated by CoffeeScript 2.6.1
try {
} catch (e) {
}
```

I understand that in cases where the catch variable is referenced outside of the catch block it must be `var` scoped as all variables in CoffeeScript are but in cases where the variable is not referenced at all outside of the block I don't feel that it should be hoisted.

Start by reproducing the CoffeeScript input `try\ncatch e` and inspect the compiler path that generates the shown JavaScript. Compare the output with the expected form, including cases where the catch variable is referenced outside the block. Done means an unused catch variable is not emitted as an extra hoisted `var`, while required outer references retain the documented behavior.

Written by the indexing model from the issue text.