Adding duckdb exceptions and result fetch methods

[chrisBirmingham]

Hello

Thank you for all your work bringing duckdb to PHP.

This PR adds three new exceptions to the extension, DuckDBException, ConnectionException and QueryException, that are thrown when duckdb fails to connect or query the database. To facilitate this, I've moved the object creation code to the bottom of each function as I've found PHP will fatal if you create the class then throw an exception.

I've also removed the USED_RET code path as I've found that running the code

$db->query('blah')->print();

Would fatal as PHP returned true instead of the result object.

Edit:

I've also updated the PR to include two new fetch methods on the result class that allows pdo style fetching of rows along with adding a usage section to the readme