handle locales-gone-missing

refs CNVS-15736

test plan:
 * set the locale for an account, course, and user to something explicit
 * remove that locale (see g/41630)
 * restart your server, and your language for each of the above should
   have reverted back to the default. also, editing the preferred
   locale for any of them should show system default again

Change-Id: Ic25290a4374a596610ffa97ad21aecd3a86f1bed
Reviewed-on: https://gerrit.instructure.com/41775
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>