At the moment, calibre-web will raise an internal 500 error if an
invalid locale identifier is passed in through the `Accept-Language`
HTTP header (for a non-logged-in user).
This breaks some crappily-built OPDS feed readers, and there's no reason
to error out so severely. This commit just gracefully falls back to `en`
if it can't parse the locale identifier.
Fixes#743 among others.
