Remove failure dependency

[chinedufn]

It's our only dependency and there aren't enough errors to be worth pulling in syn - so we should move to some hand written error impls

[chinedufn]

Eh a downside to having zero error library dependencies would be that it discourages contributors from adding new errors since there is more friction.

So maybe for now we

1. Remove failure
2. Replace it with thiserror

And then if things one day stabilize we can consider removing thiserror and replace it with hand written errors.

Alternatively we might just stick with thiserror. Who knows.

For now we'll replace failure with thiserror and that satisfies this issue for now.