error_code does not convert to result<Eigen::Matrix4d>

[pdimov]

https://godbolt.org/z/9n9xY33Pn, as reported in boostorg/json#843. Eigen::Matrix4d does have a constructor taking any T, but it's explicit, and the conversion from error_code to result<Eigen::Matrix4d> is implicit, so the explicit constructor shouldn't be considered.