[Monads] Generalize the catch_exception function

[viboes]

boost/expected/algorithms/catch_unexpected.hpp contains a function that can be generalized to all the monad_error that store an exception_ptr, as it is the case of expected<exception_ptr,T> and future<T>.

An alternative is to define this function for the error themselves, but then the user must first get the error and then do the catch_exception.

[viboes]

See #48. Once we have a MonadException concept, we can implement MonadException operations as no member functions of expected<exception_ptr,T>, given that expected<exception_ptr,T> allows to retrieve the stored exception_ptr with the error() function.

[viboes]

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