cmd/gomobile: say why a version is unknown
cmd/gomobile: say why a version is unknown

Fixes golang/go#13238

Change-Id: Ib44f9833891d721843a0fa255d9a86f1c7d43902
Reviewed-on: https://go-review.googlesource.com/16929
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile