Add error_handler to asio_server

[stephenberry]

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[stephenberry]

@gjb1002, What do you think about this solution? It adds a std::function<void(const std::string&)> error_handler{}; to provide a callback for handling errors. I think it needs to pass a std::string through to be generic.

It is important to realize that anything within the callback should be thread safe, as multiple threads could be calling this.

[gjb1002]

@stephenberry thanks for fix, looks like it resolves the issues I raised!