Added a check when forbidden is returned when validating if a key exists in a S3 Bucket.

Author: mdupras

src/ImageSharp.Web.Providers.AWS/Providers/AWSS3StorageImageProvider.cs

@@ -161,6 +161,12 @@ private async Task<bool> KeyExists(IAmazonS3 s3Client, string bucketName, string
                     return false;
                 }
 
+                // If the object exists but the client is not authorized to access it, then a "Forbidden" will be thrown.
+                if (string.Equals(e.ErrorCode, "Forbidden"))
+                {
+                    return false;
+                }
+
                 throw;
             }
         }