Checksum error is returned as a straight Exception instead of a specific error

[Christian Reis]

In bug 1404906 we run into a checksum failure likely caused by an interrupted or retried download (it's unclear what the root cause is). The exception that is raised on that error is a plain exceptions.Exception:

          File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 86, in sync
            return self.sync_products(reader, path, data, content)
          File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 318, in sync_products
            self.insert_item(item, src, target, pgree, ipath_cs)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/import_images/download_resources.py", line 200, in insert_item
            self.store, tag, checksums, size, contentsource)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/import_images/download_resources.py", line 115, in insert_root_image
            store.insert(tag, content_source, checksums, mutable=False, size=size)
          File "/usr/lib/python2.7/dist-packages/simplestreams/objectstores/__init__.py", line 166, in insert
            cksum.hexdigest(), cksum.expected))
        exceptions.Exception: unexpected checksum 'sha256' on 157dcfff1e710d216d1bad4d8fff1778d434f81ab1e8f517779fc8837c71d951 (found: 87dc32b462d1cebf466f639804ef52bb758df275fdc18fd254dc24b78344fd3d expected: 157dcfff1e710d216d1bad4d8fff1778d434f81ab1e8f517779fc8837c71d951)

Could we get a proper UnexpectedChecksumError exception instead?