Arranging JERRY_UNIMPLEMENTED macro usages and adding comments to each invocation of the macro.

src/libecmaobjects/ecma-gc.c

@@ -393,7 +393,7 @@ ecma_gc_mark (ecma_object_t *object_p, /**< start object */
           case ECMA_INTERNAL_PROPERTY_NUMBER_INDEXED_ARRAY_VALUES: /* a collection of ecma-values */
           case ECMA_INTERNAL_PROPERTY_STRING_INDEXED_ARRAY_VALUES: /* a collection of ecma-values */
           {
-            JERRY_UNIMPLEMENTED();
+            JERRY_UNIMPLEMENTED("Indexed array storage is not implemented yet.");
           }
 
           case ECMA_INTERNAL_PROPERTY_PROTOTYPE: /* the property's value is located in ecma_object_t

src/libecmaobjects/ecma-helpers-string.c

@@ -306,7 +306,8 @@ ecma_copy_or_ref_ecma_string (ecma_string_t *string_desc_p) /**< string descript
     }
     else
     {
-      JERRY_UNIMPLEMENTED ();
+      JERRY_UNIMPLEMENTED ("Unimplemented copy operation for strings, "
+                           "stored in heap chunks or number, or concatenated strings.");
     }
   }