Better error messages for .*Async methods when not using DbConnection (#986)

* Better error messages for .*Async methods when not using  DbConnection

Perhaps in v2 we move these to DbConnection itself ratherr than IDbConnection for the extensions to eliminate the case, but that's breaking...for now we can at least throw a better error. See #757 for details.

* Tweak errors to allow open IDbConnections and those generating DbCommands

Same checks but a bit more lenient with more specific messaging.

* Tweaking error messages and supporting IDbConnection implementations … (#987)