Review all `noexcept` code.

[1uc]

One example is: https://github.com/BlueBrain/HighFive/blob/f169f384040b8c69397d2f514d53aa47e0786eea/include/highfive/bits/H5Path_traits.hpp#L28

The issue is that we can't check if the object is valid before returning the file, because if we raise an exception, we'd terminate the program. Since noexcept is part of the API, we need to change it before v3.