Fix confusing error message if depthSlice is required but missing

Bug: 465787218
Change-Id: I55f803bd945990a96217271ef67e93336bfcc54a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/278475
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn/native/CommandEncoder.cpp

diff --git a/src/dawn/native/CommandEncoder.cpp b/src/dawn/native/CommandEncoder.cpp
index 6a0d1b7..60c777c 100644
--- a/src/dawn/native/CommandEncoder.cpp
+++ b/src/dawn/native/CommandEncoder.cpp
@@ -511,7 +511,7 @@
     }
 
     DAWN_INVALID_IF(depthSlice == wgpu::kDepthSliceUndefined,
-                    "depthSlice (%u) for a 3D attachment (%s) is undefined.", depthSlice,
+                    "depthSlice is required for a 3D attachment (%s), but it is undefined.",
                     attachment);
 
     const Extent3D& attachmentSize = attachment->GetSingleSubresourceVirtualSize();