Fix all GCC 10 warnings

Change-Id: Ibabab265e734a4a421a38ab586e7e11905fa5da1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/64740
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/dawn_native/ErrorScope.cpp

@@ -29,6 +29,7 @@ namespace dawn_native {
                 case wgpu::ErrorFilter::OutOfMemory:
                     return wgpu::ErrorType::OutOfMemory;
             }
+            UNREACHABLE();
         }
 
     }  // namespace