Add data store failure detection in DICOM storage service

[Enet4]

Should resolve #721.

Caveats:

• it's an SDK breaking change
• reports success whenever at least one storage provider succeeds, meaning that others could have failed when there is more than one. The scenario of having multiple storage providers at once is not that well supported anyway.

Summary

• [sdk] Change signature of StorageInterface#store(DicomObject,Object...) so it throws IOException
• [sdk] Adjust documentation of StorageInterface a bit
• Detect data storage failure in DicomStorage