Update Error_monad tutorial

The following discussion from metastatedev/tezos!164 (merged) should be addressed:

• @rafoo_ started a discussion: (+1 comment)

  For v >>=? fun x -> return x' ----> v >>|? fun x -> x', I think it raises a bit the barrier of entry to the codebase; at least I think that the error monad tutorial at http://tezos.gitlab.io/developer/error_monad.html should be updated to explain the mapping operators and promote the new style.

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When updating the tutorial, include a section that explains that result can be seen as control-flow (with >>? and >>=? being short syntax for that control) or some data (it can be matched on, etc.). This is similar to exceptions being a control flow mechanism but also data that can be stored, matched, etc. Explain the corrspondance too.