DM: support non-fatal errors

dm/DMSrcSink.cpp

 #include "DMSrcSink.h"
 #include "SamplePipeControllers.h"
 #include "SkCommonFlags.h"
 #include "SkCodec.h"
 #include "SkDocument.h"
 #include "SkError.h"
 #include "SkMultiPictureDraw.h"
 #include "SkNullCanvas.h"
 #include "SkOSFile.h"
 #include "SkPictureRecorder.h"
@@ skipping 74 matching lines @@
     }
     // Decode subsets.  This is a little involved.
     SkAutoTDelete<SkMemoryStream> stream(new SkMemoryStream(encoded));
     SkAutoTDelete<SkImageDecoder> decoder(SkImageDecoder::Factory(stream.get()));
     if (!decoder) {
         return SkStringPrintf("Can't find a good decoder for %s.", fPath.c_str());
     }
     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.
     if (fDivisor > w || fDivisor > h) {
         return SkStringPrintf("divisor %d is too big for %s with dimensions (%d x %d)",
                               fDivisor, fPath.c_str(), w, h);
     }
     const int subsetWidth  = w / fDivisor,
               subsetHeight = h / fDivisor;
     for (int y = 0; y < h; y += subsetHeight) {
@@ skipping 405 matching lines @@
             surfaces.unrefAll();
             return "";
         }
         SkISize size() const SK_OVERRIDE { return fSize; }
         Name name() const SK_OVERRIDE { sk_throw(); return ""; }  // No one should be calling this.
     } proxy(fW, fH, pic, src.size());
     return fSink->draw(proxy, bitmap, stream, log);
 }
 
 }  // namespace DM