Removed fallthrough to %n for unknown winmsg escapes

I'm not sure why this was originally the case, but it does not make sense to
me that any undefined escape should implicitly mean %n; instead, any
undefined escapes will do nothing and output nothing (and consequently not
set the condition flag).

(It looks like the fallthrough may have been added in 80a04457 as part of a
larger commit, so the rationale isn't necessarily clear; I do not much feel
like researching it at this point, so I will discus it with amade.)