Fix railgun crash on XP SP3

Fix issue where the railgun API was relyling on FormatMessage returning a
valid pointer when the error is `ERROR_SUCCESS`. On some platforms, such
as XP SP3, the function would return a NULL pointer for this case. This
fix makes sure that in the case of a NULL pointer the error message is set
to a value that matches that found on other platforms.

[FixRM 8505]

/cc @Meatballs1