exit right out when trying to get fx files for OBS

[valeriupredoi]

Addresses the issue with OBS datasets not having fx files (well, at least most of them) and reported in #294

CAVEAT this is a guillotine treatment that is not sensitive to the OBS datasets that may have fx files and should also be always used with always_use_ne_mask: True as arg in the masking preprocessor

Up for discussion 💾

[valeriupredoi]

@mattiarighi @hb326 @rswamina you guys have a ton of experience with OBSes - which ones have fx data? If none then we can safely leave the cut-throat approach from here, if otherwise we need to plug in something more fancy 🍺

[mattiarighi]

At the moment only ERA-Interim and NSIDC-0116.
But other ones are coming, e.g. #1042.

[valeriupredoi]

At the moment only ERA-Interim and NSIDC-0116.
But other ones are coming, e.g. #1042.

damn! thought I might get out easy 😁 OK we need a dedicated issue for that then

[valeriupredoi]

let's plug this in for now so we have a somewhat decent functionality and then concern about the others later

[valeriupredoi]

#297 supersedes this so am gonna close w/o merge