Separate device lost from internal errors.

The effect to the user is the same, the Dawn device gets lost. However
we need to make the difference internally because when the backend
device is lost we can clean up immediately. On the contrary on internal
errors, the backend device is still alive and processing commands so we
need to gracefully shut it down.

Bug: dawn:269

Change-Id: Ie13b33a4f9ac2e1f5f98b3723d83cf1c6205c988
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/17965
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/dawn.json b/dawn.json
index d53239f..6973a94 100644
--- a/dawn.json
+++ b/dawn.json
@@ -32,7 +32,7 @@
             {"value": 0, "name": "discrete GPU"},
             {"value": 1, "name": "integrated GPU"},
             {"value": 2, "name": "CPU"},
-            {"value": 3, "name": "Unknown"}
+            {"value": 3, "name": "unknown"}
         ]
     },
     "address mode": {