Bug 1095506 - Serialize the error object when saving to the store. r=paolo

We now serialize the Download.error property when saving to the store so
that data about error conditions will survive a restart. This is
important for supporting blocked downloads with partial data that could
be unblocked.

--HG--
extra : rebase_source : 8edaff4ddc2efb056bf06e689c7abe6cba72eabf
Bug 1095506 - Clean up and rename kSerializableDownloadProperties. r=paolo

--HG--
extra : rebase_source : 5062b40701f4ebcdadef6a60197e422de51ab119