Better handling of error flags in S3 stream wrapper

PHP stream wrappers provide flags to url_stat that should be used to determine
the response of a call and whether or not it raises an error. I've updated the
Amazon S3 stream wrapper to utilize these combinations of flags to emulate the
error handling of the "http" protocol stream wrapper.

I'm also removing the "throw_exceptions" option from the stream wrapper as that
completely breaks compatibility with other stream wrappers.

Closes #268