Fix #1225 -- stop `nikola init` from crashing if invalid locales are specified.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Make nikola crash more friendly if a language cannot be found

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>
Merge pull request #1228 from getnikola/fix-1225

Fix #1225 -- stop `nikola init` from crashing if invalid locales are specified.