fix: Use variable instead of custom name joining for public bucket (#…

…1397)

I monkey patched this name for testing and the error was confusing.

src/viur/core/modules/file.py

@@ -508,7 +508,7 @@ def get_bucket(dlkey: str) -> google.cloud.storage.bucket.Bucket:
                 return _public_bucket
 
             raise ValueError(
-                f"""The bucket 'public-dot-{_PROJECT_ID}' does not exist! Please create it with ACL access."""
+                f"""The bucket '{PUBLIC_BUCKET_NAME}' does not exist! Please create it with ACL access."""
             )
 
         return _private_bucket