Need error message when archive file doesn't exist/isn't set

[benrhughes]

Currently, if you have auto-archive enabled but no archive file set (or the file doesn't exist) the options dialog opens when you try and complete an issue. This is pretty confusing, because it doesn't tell you what the problem is.

[mjdescy]

Also, there should be a warning when "automatically archive completed tasks" is set but no archive file is set.