Bug 1608100 - Distinguish (in console log messages) between failure to start font download at all vs download that fails. r=heycam

Differential Revision: https://phabricator.services.mozilla.com/D59328

--HG--
extra : moz-landing-system : lando

gfx/thebes/gfxUserFontSet.cpp

@@ -626,7 +626,7 @@ void gfxUserFontEntry::DoLoadNextSrc(bool aForceAsync) {
             }
             return;
           } else {
-            mFontSet->LogMessage(this, "download failed",
+            mFontSet->LogMessage(this, "failed to start download",
                                  nsIScriptError::errorFlag, rv);
           }
         }