Issue 3919 Use 404 rather than hard error or redirect.

This is a reworking of https://github.com/otwcode/otwarchive/pull/1531 and Stephaine   should be credited.

https://code.google.com/p/otwarchive/issues/detail?id=3933
So are we assuming that @tag is a Tag due to having been found in Tag.find_by_name?