When an upload exceeds the maximum limit, `ErrEntityTooLarge` gets
returned but is wrapped in multiple layers of errors when it is
checked. As a result, when Google Cloud Storage were used to upload
files, artifacts exceeding the maximum size would report a "500 Internal
Server Error" instead of the correct "413 Request Entity Too Large"
error message.

To fix this, we check the state of `hardLimitReader` and set the error
to `ErrEntityTooLarge` at the end of the `SaveFileFromReader` to ensure
that this error will be returned.

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/-/issues/328