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.

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extra : rebase_source : 40821e59e2b258803cde5da456cd9d0d20caa9e0
Bug 1095506 - Clean up and rename kSerializableDownloadProperties. r=paolo

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extra : rebase_source : 4ac9627e2d8d108e416e50b8746825714ff4e52e