Give a better error message for assigning from Object to anything else

Test code

``` ts
declare var x: Object;
declare var y: RegExp;
y = x;
```

A lot of people misunderstand what `Object` is. We should consider trying to elaborate the issue in some way.

```
Type 'Object' is not assignable to type 'RegExp'.
  The 'Object' type is assignable to very few other types. Did you mean to use the 'any' type instead?
```
