Ensure StorageAPI always re-raises exceptions appropriately

Due to the I/O included the exception handlers, eventlet correctly switches
greenthreads in order to not block. This has the side effect of clearing
pythons internal exception context, resulting in the reraise failing with
a confusing error message.

Fixes bug #1204539

Change-Id: I15061ed48bef09890ac4649a4705b0bf83c02a81