Again tweak detection of when an image doesn't exist.

Let's see if this fragile nonsense breaks again.

What was here though was hiding containers/podman#19930
(i.e. a real 500 error coming from a podman bug which was
being reported as the image not existing).

bowtie/_core.py

@@ -223,8 +223,17 @@ async def start(
             raise StartupFailed(name=image_name)
         except aiodocker.exceptions.DockerError as error:
             status, data, *_ = error.args  # :/
-            if data.get("cause") == "image not known" or status == 500:
+            if data.get("cause") == "image not known":
                 raise NoSuchImage(name=image_name, data=data)
+
+            try:  # GitHub Registry saying an image doesn't exist...
+                error = json.loads(data.get("message", "{}")).get("error", "")
+            except Exception:
+                pass
+            else:
+                if "403 (forbidden)" in error.casefold():
+                    raise NoSuchImage(name=image_name, data=data)
+
             raise StartupFailed(name=image_name, data=data)
 
         yield self