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:

Review URL: https://codereview.chromium.org/980333002

1 files changed, 1 insertions, 1 deletions

diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 2060ce2d68..d99f0115b3 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -92,7 +92,7 @@ Error ImageSrc::draw(SkCanvas* canvas) const {
     stream->rewind();
     int w,h;
     if (!decoder->buildTileIndex(stream.detach(), &w, &h) || w*h == 1) {
-        return "";  // Not an error.  Subset decoding is not always supported.
+        return Error::Nonfatal("Subset decoding not supported.");
     }
 
     // Divide the image into subsets that cover the entire image.