As a related question, I was also recently wondering if -verbose should be true by default:

a) Builders always pass -verbose.
b) Developers (probably?) always pass -verbose, since they want to see output from their tests.

I don't know if there are any good reasons for -verbose to ever not be true.

But in any case, I agree that download errors should be logged at the non-verbose level. Are they reported via RunLog control messages?

Right, download errors are reported by RunLog.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/tast/+/5112a07bb17003e77dc3b226102daa24971c17e4

commit 5112a07bb17003e77dc3b226102daa24971c17e4
Author: Shuhei Takahashi <nya@chromium.org>
Date: Tue Dec 18 00:52:22 2018

tast: Clarify external data file download failures.

When local_test_runner fails to download external data files, it
will write reasons to *.external-error files. Those files are read
by bundles to report more detailed errors.

BUG= chromium:914183 
TEST=fast_build.sh -T
TEST=Put a broken *.external file and run tests

Change-Id: I134b06beef1722e7e936940427aaa4e50e1f99f9
Reviewed-on: https://chromium-review.googlesource.com/1375490
Commit-Ready: Shuhei Takahashi <nya@chromium.org>
Tested-by: Shuhei Takahashi <nya@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>

[modify] https://crrev.com/5112a07bb17003e77dc3b226102daa24971c17e4/src/chromiumos/tast/runner/external_test.go
[modify] https://crrev.com/5112a07bb17003e77dc3b226102daa24971c17e4/src/chromiumos/tast/testing/test.go
[modify] https://crrev.com/5112a07bb17003e77dc3b226102daa24971c17e4/src/chromiumos/cmd/tast/run/local.go
[modify] https://crrev.com/5112a07bb17003e77dc3b226102daa24971c17e4/src/chromiumos/cmd/tast/run/local_test.go
[modify] https://crrev.com/5112a07bb17003e77dc3b226102daa24971c17e4/src/chromiumos/tast/testing/test_test.go
[modify] https://crrev.com/5112a07bb17003e77dc3b226102daa24971c17e4/src/chromiumos/tast/runner/external.go