[s3] Re-raise non-404 errors in .exists()

storages/backends/s3boto3.py

@@ -469,13 +469,11 @@ def exists(self, name):
             self.connection.meta.client.head_object(Bucket=self.bucket_name, Key=name)
             return True
         except ClientError as error:
-            if error.response.get('Error', {}).get('Code') == '404':
+            if error.response['ResponseMetadata']['HTTPStatusCode'] == 404:
                 return False
 
-            # Some other error was encountered. As `get_available_name` calls this,
-            # we have to assume the filename is unavailable. If we return true due to some
-            # other error, we'd overwrite a file.
-            return True
+            # Some other error was encountered. Re-raise it.
+            raise
 
     def listdir(self, name):
         path = self._normalize_name(self._clean_name(name))