[JS BE] Improve error message in JS BE

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt

@@ -68,7 +68,9 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
 
         if (fieldParent is IrClass && field.isEffectivelyExternal()) {
             // External fields are only allowed in external enums
-            assert(fieldParent.isEnumClass)
+            assert(fieldParent.isEnumClass) {
+                "${field.render()} in non-external class ${fieldParent.render()}"
+            }
             return JsNameRef(
                 field.getJsNameOrKotlinName().identifier,
                 context.getNameForClass(fieldParent).makeRef()

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/utils/NameTables.kt

@@ -333,7 +333,7 @@ class NameTables(
         val name = memberNames.names[signature] ?: mappedNames[mapToKey(signature)]
 
         require(name != null) {
-            "Can't find name for member field $field"
+            "Can't find name for member field ${field.render()}"
         }
         return name
     }