DM: support non-fatal errors

DM: support non-fatal errors

Tasks that produce a non-fatal error will bail out before writing their output to
disk and hash to dm.json, but not count as failures.

This also makes true failures bail out before writing their results.  If the DM
program failed, we probably don't want to triage that image result.

We use this new feature first to skip image subset decoding when we detect it's
not supported.  Here's a snippet of an example run, where in this case only
.webp are subset decodable:

...
(  15MB    12) 172µs	8888 subset color_wheel.jpg (skipped: Subset decoding not supported.)
(  15MB    11) 9.05ms	8888 subset randPixels.webp
(  16MB    10) 863µs	8888 subset baby_tux.png (skipped: Subset decoding not supported.)
...

Only outputs corresponding to the .webp show up, both on disk and in the .json.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/4089ef7c982592fadeec441438c551ffb4c746be

[mtklein_C, 2015-03-05 16:27:11]

mtklein@chromium.org changed reviewers:
+ scroggo@google.com

[mtklein_C, 2015-03-05 16:27:12]

I think this should let us skip uninteresting decodes ( return
Error::Nonfatal("uninteresting"); ) without having to have the Sinks worry about
clearing the bitmaps.  We'll just ignore it anyway.

[scroggo, 2015-03-05 16:32:13]

On 2015/03/05 16:27:12, mtklein_C wrote:
> I think this should let us skip uninteresting decodes ( return
> Error::Nonfatal("uninteresting"); ) without having to have the Sinks worry
about
> clearing the bitmaps.  We'll just ignore it anyway.

LGTM

[scroggo, 2015-03-05 16:33:55]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2015-03-05 16:34:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/980333002/1

[mtklein, 2015-03-05 16:34:58]

Patchset #2 (id:20001) has been deleted

[mtklein, 2015-03-05 16:35:02]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-03-05 16:35:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/980333002/1

[commit-bot: I haz the power, 2015-03-05 16:40:31]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/4089ef7c982592fadeec441438c551ffb4c746be