Some sub-systems like btcwallet will return an error from the database,
but they won't properly wrap it. As a result, we were unable to actually
catch the serialization errors in the first place. To work around this,
we'll now attempt to parse the error string directly.
In this commit, we use exhaustive build tags to ensure that we can
always build the `sqlbase` package, independent of the set build tags.
To do this, we move the type declarations _into_ the parsing functions.
This then allows us to create two versions for each db: with the db, and
without it.
To avoid a module tag round trip to get this working, we use a local
replace for now. Once this is merged in, we can do the tag (along side
rc3), then remove the replace.
