Bug 1497029 showed that symbol files were truncated. This turns out to happen because for some reason dump_syms fails (haven't dug why yet), but the symbols dumper doesn't care that it failed, so it goes further by shipping the incomplete symbol files. We should prevent this from happening silently.

I can't remember why we never did this. I think we might have had some errors during normal operation that were difficult to work around in the past? Hopefully that's no longer the case, because this does result in hard-to-diagnose errors further down the line.

Pushed by mh@glandium.org: https://hg.mozilla.org/integration/autoland/rev/8cca84d74504 Make symbolstore.py fail when dump_syms fails r=froydnj