Better error reporting a debugging

[hason]

We should throw exceptions with exact position in source file (also useful for debugging).

[cundd]

We added a custom exception class to carry more information in the exception object (https://github.com/iresults/scssphp/blob/master/Exception/ScssException.php).

Unfortunately we didn't manage to find the line in the SASS file which contains the failure.

[leafo]

This is actually pretty easy to fix I think, you can do what I did in lessphp:

https://github.com/leafo/lessphp/blob/master/lessc.inc.php#L3287

When parsing into AST, every time you add a new node, you just insert the position of the buffer in as -1 property of the node. Then later on when compiling you set the most recent position and when an exception is thrown it can check that value for the position.

[leafo]

Okay should be good now