Turn `Replacing existing passage "..." with duplicate.` into an error

[BawdyInkSlinger]

If you have duplicate passage names in your project, I think tweego should report that as an error. I expect, for backwards compatibility, this won't be possible, but it would be convenient to have a flag or something that could make this warning into an error. In my project, it would always be a mistake to get this warning, but with the noise of my build scripts, it's difficult to notice with my eyes when it prints out. I'd like some way for my script to fail.

I'm developing on windows with cygwin.

[tmedwards]

Not an unreasonable request. I should be able to add something like this to the next minor release.

[BawdyInkSlinger]

Thanks!