i18n: use POTFILES.in as fallback if there's no POTFILES in po dir
i18n: use POTFILES.in as fallback if there's no POTFILES in po dir

In autotools POTFILES is generated at configure time from POTFILES.in,
but Meson only looks for a po/POTFILES in the source directory, which
is awkward when trying to maintain both build systems in parallel.
Instead just use POTFILES.in as fallback if it exists but POTFILES
does not. Also print an error if neither exists.

Fixes #818

trhd/meson,QuLogic/meson,MathieuDuponchelle/meson,rhd/meson,becm/meson,mesonbuild/meson,aaronp24/meson,jeandet/meson,jeandet/meson,ernestask/meson,pexip/meson,jeandet/meson,thiblahute/meson,rhd/meson,ernestask/meson,wberrier/meson,thiblahute/meson,wberrier/meson,rhd/meson,ernestask/meson,jpakkane/meson,ernestask/meson,aaronp24/meson,becm/meson,pexip/meson,jpakkane/meson,fmuellner/meson,MathieuDuponchelle/meson,wberrier/meson,jpakkane/meson,rhd/meson,aaronp24/meson,fmuellner/meson,pexip/meson,rhd/meson,thiblahute/meson,jeandet/meson,jpakkane/meson,MathieuDuponchelle/meson,fmuellner/meson,pexip/meson,ernestask/meson,jpakkane/meson,fmuellner/meson,becm/meson,mesonbuild/meson,MathieuDuponchelle/meson,mesonbuild/meson,thiblahute/meson,pexip/meson,QuLogic/meson,centricular/meson,aaronp24/meson,mesonbuild/meson,becm/meson,ernestask/meson,aaronp24/meson,becm/meson,becm/meson,becm/meson,jpakkane/meson,thiblahute/meson,centricular/meson,trhd/meson,centricular/meson,jeandet/meson,pexip/meson,centricular/meson,QuLogic/meson,becm/meson,trhd/meson,mesonbuild/meson,wberrier/meson,aaronp24/meson,centricular/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,wberrier/meson,thiblahute/meson,mesonbuild/meson,rhd/meson,MathieuDuponchelle/meson,pexip/meson,mesonbuild/meson,ernestask/meson,centricular/meson,jpakkane/meson,QuLogic/meson,fmuellner/meson,mesonbuild/meson,centricular/meson,aaronp24/meson,jeandet/meson,mesonbuild/meson,rhd/meson,centricular/meson,wberrier/meson,becm/meson,aaronp24/meson,rhd/meson,fmuellner/meson,jeandet/meson,QuLogic/meson,QuLogic/meson,becm/meson,aaronp24/meson,thiblahute/meson,ernestask/meson,trhd/meson,pexip/meson,trhd/meson,MathieuDuponchelle/meson,ernestask/meson,MathieuDuponchelle/meson,fmuellner/meson,mesonbuild/meson,wberrier/meson,trhd/meson,trhd/meson,pexip/meson,MathieuDuponchelle/meson,jpakkane/meson,trhd/meson,MathieuDuponchelle/meson,thiblahute/meson,thiblahute/meson,fmuellner/meson,trhd/meson,wberrier/meson,QuLogic/meson,pexip/meson,fmuellner/meson,jeandet/meson,jeandet/meson