cmd/coordinator: say when reverse pool is empty
cmd/coordinator: say when reverse pool is empty

Change-Id: Icd9933d42a1d0b04703ae23827532e8c2eecc4c7
Reviewed-on: https://go-review.googlesource.com/9451
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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