[autotest] Clarify run_suite message when no hosts exist in the pool.

There's a simpler way to phrase the error, and logging the rpc_interface
provides no useful information.

BUG=chromium:377212
DEPLOY=run_suite
TEST=None

Change-Id: I705a4d9a9e9831ed82b53631642e7590c070eca5
Reviewed-on: https://chromium-review.googlesource.com/202360
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
Tested-by: Alex Miller <milleral@chromium.org>
1 file changed