[glsl][ir] Add context to unreachable message

Add some context for the `TINT_UNREACHABLE` for a storage format in
read-write which is disallowed by ES.

Bug: 42251044
Change-Id: I1f880ccca57b445f5a18c79709da7e886c3f5c5c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/208134
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/src/tint/lang/glsl/writer/printer/printer.cc b/src/tint/lang/glsl/writer/printer/printer.cc
index d02ccce..c4cc0fd 100644
--- a/src/tint/lang/glsl/writer/printer/printer.cc
+++ b/src/tint/lang/glsl/writer/printer/printer.cc
@@ -730,7 +730,7 @@
                             case core::TexelFormat::kR32Uint:
                                 break;
                             default:
-                                TINT_UNREACHABLE();
+                                TINT_UNREACHABLE() << "invalid texel format for read-write";
                         }
                     }
                     break;