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

src/libcoreint/interpreter.c

@@ -439,7 +439,7 @@ run_int_loop (int_data_t *int_data)
     if (completion.type == ECMA_COMPLETION_TYPE_BREAK
         || completion.type == ECMA_COMPLETION_TYPE_CONTINUE)
     {
-      JERRY_UNIMPLEMENTED ();
+      JERRY_UNIMPLEMENTED ("break and continue on labels are not supported.");
 
       continue;
     }

src/libcoreint/opcodes-native-call.c

@@ -141,7 +141,7 @@ opfunc_native_call (opcode_t opdata, /**< operation data */
 #if CONFIG_ECMA_CHAR_ENCODING == CONFIG_ECMA_CHAR_ASCII
         __printf ("%s\n", (char*) zt_str_p);
 #elif CONFIG_ECMA_CHAR_ENCODING == CONFIG_ECMA_CHAR_UTF16
-        JERRY_UNIMPLEMENTED ();
+        JERRY_UNIMPLEMENTED ("UTF-16 support is not implemented.");
 #endif /* CONFIG_ECMA_CHAR_ENCODING == CONFIG_ECMA_CHAR_UTF16 */
 
         mem_heap_free_block (zt_str_p);