balrog check on nightly broke when asan nightlies were added

Fairly recently we started providing nightly updates for ASan builds (https://bugzilla.mozilla.org/show_bug.cgi?id=1403548). In doing this, we added a Linux_x86_64-gcc3-asan platform to the Firefox-mozilla-central-nightly-latest blob, which breaks the assumption that de is present in locales blocks at

PollBot/pollbot/tasks/balrog.py

Line 23 in a3b8afe

platform_info = platforms[platform]['locales']["de"]

, since we do not provide localized ASan builds.

2:20 PM <peterbe> Callek: Do you and Ben know what to do to resolve that between the two of you? I
read the issue but it doesn’t mean much to me. I’m just trying to pretend to know what Pollbot is when
mago is on PTO.
2:21 PM <Callek> peterbe: my *guess* is to scan all locales, get the buildid's for all locales for all
platforms, and then compare them. rather than hardcode a single locale to check (since, as said, asan
doesn't have de right now, only en-US)
2:21 PM <Callek> alternatively we could exclude asan from the delivery dashboard entirely and hardcode
it as an excluded platform
2:21 PM <Callek> or there may be another option, maybe only test de and skip if de isn't in the platform
2:22 PM <peterbe> Ok. Sorry for sounding like I’m directing work at you but would you mind jotting
down those same thoughs on the issue?
2:22 PM <Callek> peterbe: those are all my own brief brainstormed ideas, ben may have others or a
preference, and I'm not sure who 'owns' this portion of pollbot and/or who the consumers are to know if
the busted pollbot right now is a problem until <someone> is back from a PTO
2:23 PM <Callek> sure, I'll go the lazy-mans way and copy/paste from IRC though

I'm not exactly sure what to suggest because I don't fully understand the reasoning behind just checking de, but I would guess that checking all locales is too expensive. Excluding build targets matching "asan" might be the right thing to do, if RelMan doesn't care about asan.