Tweak AsyncConnection.transation to return custom error types
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Currently,
AsyncConnection.transactionis hard-coded to only allow a closure that returns anAsyncResult. This PR takes a cue fromdiesel::Connection.transactionand allows it to return aResultwith any error type-- as long as it implementsFrom<diesel::result::Error>.To implement it, I added a small utility type (
TransactionError<E>) that simply wraps an error type, the unwraps it after the transaction completes. We need this so we can go fromdiesel::result::ErrortoAsyncError. There may be better ways to solve this, but this was the first solution that came to mind.I found that allowing any error type made it a lot easier to work with transactions (especially when using
anyhow), and this seems like a net-positive from an ergonomics perspective. I'm open to suggestions for other possible improvements for this PR though!