Make UploadInterruptedException visible (#12070)

When another threads interrupts the Upload, the UploadInterruptedException is
thrown. The respective StreamVariable will receive
streamingFailed(StreamingErrorEvent event) which contains the causing exception
via event.getException(). When the UploadInterruptedException is not public, it
is impossible to react on it in the StreamVariable (e.g. treating an
IOException differently from an UploadInterruptedException).

Change-Id: I9098e70e6750c420fcc0978340544e545e237b26