Be more verbose about missing sequences and files

as requested in #3039 although it does not solve all problems. The design decision to keep sequence definitions separate from game rules might be appealing for mods which have them all set up correctly, but modders usually forget about them when adding new unit rules. Now they get a warning immediately on startup that something will go wrong when the asset is loaded so that you don't have to click through the build palette and test build every unit to check for related crashes.