Define a custom exception type for IO errors thrown when opening source images, and catch that in InageNode.render so that we're not masking unrelated IOErrors elsewhere in the get_rendition process

wagtail/wagtailimages/models.py

@@ -1,7 +1,7 @@
 import os.path
 import re
 
-from six import BytesIO
+from six import BytesIO, text_type
 
 from taggit.managers import TaggableManager
 
@@ -28,6 +28,13 @@ from wagtail.wagtailimages.rect import Rect
 from wagtail.wagtailadmin.utils import get_object_usage
 
 
+class SourceImageIOError(IOError):
+    """
+    Custom exception to distinguish IOErrors that were thrown while opening the source image
+    """
+    pass
+
+
 def get_upload_to(instance, filename):
     folder_name = 'original_images'
     filename = instance.file.field.storage.get_valid_name(filename)
@@ -178,7 +185,15 @@ class AbstractImage(models.Model, TagSearchable):
             # If we have a backend attribute then pass it to process
             # image - else pass 'default'
             backend_name = getattr(self, 'backend', 'default')
-            generated_image = filter.process_image(file_field.file, backend_name=backend_name, focal_point=self.get_focal_point())
+
+            try:
+                image_file = file_field.file  # triggers a call to self.storage.open, so IOErrors from missing files will be raised at this point
+            except IOError as e:
+                # re-throw this as a SourceImageIOError so that calling code can distinguish
+                # these from IOErrors elsewhere in the process
+                raise SourceImageIOError(text_type(e))
+
+            generated_image = filter.process_image(image_file, backend_name=backend_name, focal_point=self.get_focal_point())
 
             # generate new filename derived from old one, inserting the filter spec and focal point key before the extension
             if self.has_focal_point():

wagtail/wagtailimages/templatetags/wagtailimages_tags.py

@@ -1,6 +1,6 @@
 from django import template
 
-from wagtail.wagtailimages.models import Filter
+from wagtail.wagtailimages.models import Filter, SourceImageIOError
 
 register = template.Library()
 
@@ -53,10 +53,10 @@ class ImageNode(template.Node):
 
         try:
             rendition = image.get_rendition(self.filter)
-        except IOError:
+        except SourceImageIOError:
             # It's fairly routine for people to pull down remote databases to their
             # local dev versions without retrieving the corresponding image files.
-            # In such a case, we would get an IOError at the point where we try to
+            # In such a case, we would get a SourceImageIOError at the point where we try to
             # create the resized version of a non-existent image. Since this is a
             # bit catastrophic for a missing image, we'll substitute a dummy
             # Rendition object so that we just output a broken link instead.