map.js: add fallback for missing base layers
map.js: add fallback for missing base layers

shadowoneau/skylines,RBE-Avionik/skylines,skylines-project/skylines,TobiasLohner/SkyLines,shadowoneau/skylines,TobiasLohner/SkyLines,snip/skylines,skylines-project/skylines,RBE-Avionik/skylines,Harry-R/skylines,Harry-R/skylines,snip/skylines,TobiasLohner/SkyLines,Turbo87/skylines,kerel-fs/skylines,skylines-project/skylines,Harry-R/skylines,Turbo87/skylines,kerel-fs/skylines,Turbo87/skylines,shadowoneau/skylines,RBE-Avionik/skylines,Turbo87/skylines,snip/skylines,kerel-fs/skylines,Harry-R/skylines,RBE-Avionik/skylines,skylines-project/skylines,shadowoneau/skylines