Distill generic part from error monad

Context

This MR separates the different components of the error monad into generic ones (that do not use a hard-coded error type) and the specilaised ones (that use the extensible type error = ..). These changes are propagated to Lwtreslib too.

The long-term goal is (1) cleaner code, (2) being able to use the power of the error monad for either the built-in error type or for more local errors. This MR is one step in this direction. See !1872 (closed) for other MRs that take other steps in the same direction.

Reviewers

@mbouaziz @Saroupille