Improve error handling ergonomics for call

[Ademan]

This change I think it improves the ergonomics of handling custom errors significantly.

From the tests you can see that it's fairly unintrusive overall, it eliminates the need for lots of .map_err(|e| e.into())s and eliminates the need for downcasting the Error::Other variant, as well as consolidating Error::Rusqlite and Error::Other into Error::Error which is generic.

In particular look at test_ergonomic_errors to see how it improves error ergonomics.

I understand if it's too big of a change though.