No error check in Imagick::__toString()

[Danack]

	image_contents = MagickGetImageBlob(intern->magick_wand, &image_size);
	if (!image_contents) {
		return;
	}

And apparently it does fail...

[bukka]

The getImageBlog is fixed but there is still no check in __toString so will repurpose this.

[bukka]

Also we should probably trow in __toString on PHP versions that support exception there.