Improved error message when an assocArray can't be created

[Zevenberge]

I was trying to filter out an assocArray and transforming the result back into an assocArray again for debugging in dub, and got the error message "assocArray: argument must be a range of tuples". Expanded it to include the actual type.

Also I noticed that the static assert was never hit in a test, so I've written a trivial test for it.

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[thewilsonator]

Thanks.