Catch PIL imread problems early and generate error message

skimage/io/_plugins/pil_plugin.py

@@ -29,7 +29,11 @@ def imread(fname, dtype=None):
 
     """
     im = Image.open(fname)
-    return pil_to_ndarray(im, dtype)
+    try:
+        return pil_to_ndarray(im, dtype)
+    except IOError:
+            raise ValueError('Could not load "%s": make sure you have library '
+                'support for "%s" files' % (fname, im.format))
 
 
 def pil_to_ndarray(im, dtype=None):
@@ -55,6 +59,8 @@ def pil_to_ndarray(im, dtype=None):
         im.shape = shape[::-1]
     elif 'A' in im.mode:
         im = im.convert('RGBA')
+    # this will raise an IOError if the file is not readable
+    im.getdata()[0]
     im = np.array(im, dtype=dtype)
     if fp is not None:
         fp.close()