Slightly rewrite how conflicting activation errors are processed.
Slightly rewrite how conflicting activation errors are processed.

I'm not sure why the original code partitioned the errors in the way
that it did. I think it is better to exhaustively match all the reasons,
so that when new reasons are added, it will be clear that this code
needs to be updated. I also think it simplifies the code a little.

nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,tshepang/cargo,tshepang/cargo,rustyhorde/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,eminence/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,nipunn1313/cargo