Only set LOAD_TRUNCATED_IMAGES when if the Image open fails.

Document which PIL issues this works around.

node_helpers.py

-from PIL import Image, ImageFile
+from PIL import Image, ImageFile, UnidentifiedImageError
 
 def conditioning_set_values(conditioning, values={}):
     c = []
@@ -11,14 +11,15 @@ def conditioning_set_values(conditioning, values={}):
     return c
 
 def open_image(path):
-    try :
-        ImageFile.LOAD_TRUNCATED_IMAGES = False
-        img = Image.open(path)
+    prev_value = None
 
-    except:
+    try:
+        img = Image.open(path)
+    except (UnidentifiedImageError, ValueError): #PIL issues #4472 and #2445
+        prev_value = ImageFile.LOAD_TRUNCATED_IMAGES
         ImageFile.LOAD_TRUNCATED_IMAGES = True
         img = Image.open(path)
-        
     finally:
-        ImageFile.LOAD_TRUNCATED_IMAGES = False
+        if prev_value is not None:
+            ImageFile.LOAD_TRUNCATED_IMAGES = prev_value
         return img