New feature: improve diagnostics for parse errors

[mikkorantalainen]

In case of parse errors the Parser::parse() will throw Exception or UnexpectedTokenException depending on error type.

However, this is not enough to create meaningful diagnostics about the problem. For example a CSS document such as this

@foo (something=another))
{
  bar { zoo: 1; }
}

will cause parse() to throw \Exception with a message Unopened { where the real problem is an extra closing parenthesis before first '{'.

In best case, there would be much more exception classes for each specific parse error, but I think even adding a simple public method to Parser.php would improve things a lot:

    /**
     * @return int byte offset for current parser position
     *  In case the parse() throws an exception, this method can be
     *  used to approximate the error location.
     */
    public function getPosition()
    {
        return $this->iCurrentPosition;
    }