Fix openslide.open_slide() suboptimalities

open_slide() was calling can_open() before opening the slide, exactly
as the documentation says not to.  After failing to open a slide, it
was swallowing the error.

Author: bgilbert

openslide/__init__.py

@@ -352,9 +352,9 @@ def open_slide(filename):
 
     Return an OpenSlide object for whole-slide images and an ImageSlide
     object for other types of images."""
-    if OpenSlide.can_open(filename):
+    try:
         return OpenSlide(filename)
-    else:
+    except OpenSlideUnsupportedFormatError:
         return ImageSlide(filename)