Improve SetMoveType error messages

https://github.com/beyond-all-reason/RecoilEngine/blob/b70976ab63af3df0de4a9f522cd3f95df918444a/rts/Lua/LuaSyncedMoveCtrl.cpp#L766-L770

This says "incompatible movetype key for %s" but actually it can also fail if something is wrong with the value, most commonly if you have a good key but pass nil.

Start at rts/Lua/LuaSyncedMoveCtrl.cpp lines 766-770 and inspect how SetMoveType reports failures for the key and value. Update the error wording so it distinguishes an incompatible key from an invalid value such as nil, then run the relevant existing tests or checks for Lua move control if available.

Written by the indexing model from the issue text.