Rename namespace (i18n -> Localizer) for ExceptionLocalizer

more inline with php naming conventions, and makes it easier to
have repo-specific stuff in Wikibase/Repo/Localizer.

Change-Id: Ic0b7916b900ce747b166e49a8a40ab3f9acd098d
Merge "Rename namespace (i18n -> Localizer) for ExceptionLocalizer"