Improve parsing error message (T6710) 

> Issue originally made by @markelog

Ref https://github.com/babel/babel/issues/3774
### 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" 
