Add more context to unknown expression error

Change-Id: I16bc118f5d3ec6536890404748fd06eb89a4433e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20142
Reviewed-by: David Neto <dneto@google.com>

src/writer/spirv/builder.cc

@@ -207,7 +207,7 @@ uint32_t Builder::GenerateExpression(ast::Expression* expr) {
     return GenerateIdentifierExpression(expr->AsIdentifier());
   }
 
-  error_ = "unknown expression type";
+  error_ = "unknown expression type: " + expr->str();
   return 0;
 }