Improve parsing error message (T6710)

> Issue originally made by @markelog
### Input code

``` js
require("babylon").parse("2++");
```
### Description

Will throw a error - 

"SyntaxError: Assigning to rvalue (1:0)"

Which is pretty cryptic, parsers like esprima, or any other JS engine will show more meaningful message  - "Invalid left-hand side expression in postfix operation" 
