Improve error handling in :mod:`sqlite3` and raise more accurate exceptions.

* :exc:`MemoryError` is now raised instead of :exc:`sqlite3.Warning` when memory is not enough for encoding a statement to UTF-8 in ``Connection.__call__()`` and ``Cursor.execute()``.
* :exc:`UnicodEncodeError` is now raised instead of :exc:`sqlite3.Warning` when the statement contains surrogate characters in ``Connection.__call__()`` and ``Cursor.execute()``.
* :exc:`TypeError` is now raised instead of :exc:`ValueError` for non-string script argument in ``Cursor.executescript()``.
* :exc:`ValueError` is now raised for script containing the null character instead of truncating it in ``Cursor.executescript()``.
* Correctly handle exceptions raised when getting boolean value of the result of the progress handler.
* Add many tests covering different corner cases.