app/build/handler: error out telling which version we got
app/build/handler: error out telling which version we got

Change-Id: I789d420279fbc1c27fec02ef4c7556b2f9997b1c
Reviewed-on: https://go-review.googlesource.com/17290
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

proppy/build,golang/build,proppy/build,proppy/build,golang/build,proppy/build,golang/build,golang/build