[Toybox] "bad xform" not very helpful

[enh]

the kernel folks are trying to use `tar --transform` and getting the "bad
xform" error that's not really helping them much. it seems like it would be
hard to list the _relevant_ sed expression(s), so perhaps just showing the
specific filename we tried that failed is a better choice?

context here:
https://android-review.googlesource.com/c/kernel/build/+/2151397/3//COMMIT_MSG#10
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