C++ error message returned by `what` must be NUL-terminated. However,
the current copy function only copied the characters, but didn't add
the NUL. Allocate one more byte and set it to NUL.

When the error message is copied, the allocation may fail. This would
terminate the process, since it's called from a `noexcept` function.

Make the allocation `nothrow` and handle `nullptr` in `what()` method
appropriately, returning a generic message saying that the message
cannot be allocated.

This also prevents a crash if `what()` is called on a moved object
(which is technically UB, but still better not to crash).