Improve the cache miss exception context.

CHANGELOG.md

@@ -9,6 +9,10 @@
 
 ### Improvements
 - More untiled tiff files are handles by the bioformats reader (#569)
+- Expose a concurrent option on endpoints for converting images (#583)
+
+### Changes
+- Exceptions on cached items are no longer within the KeyError context (#584)
 
 ## Version 1.4.3
 

large_image/cache_util/cache.py

@@ -181,11 +181,15 @@ def __call__(cls, *args, **kwargs):  # noqa - N805
         key = cls.__name__ + ' ' + key
         with cacheLock:
             try:
-                instance = cache[key]
+                return cache[key]
             except KeyError:
-                instance = super().__call__(*args, **kwargs)
-                cache[key] = instance
-                instance._classkey = key
+                # By passing and handling the cache miss outside of the
+                # exception, any exceptions while trying to populate the cache
+                # will not be reported in the cache exception context.
+                pass
+            instance = super().__call__(*args, **kwargs)
+            cache[key] = instance
+            instance._classkey = key
 
         return instance