Add singular `Web5Error` for error cases

Originally brought up here https://github.com/TBD54566975/web5-rs/issues/277

Further hinted at here https://github.com/TBD54566975/web5-rs/pull/283#discussion_r1720046966

Basically, we should vastly constrict our error cases such that the consuming code's DX is just a simple `Web5Error` which they come to immediately identify as originating from our sdk