enhance for "if not exists"

1. create an edgetype with prop

> create edge choose(grade int)

2. create the edgetype without any prop using if not exists

> create edge if not exists choose()
Execution succeeded (Time spent: 12.129/13.466 ms)

3. check the schema of choose

describe edge choose;
================
| Field | Type |
================
| grade | int  |
----------------
Got 1 rows (Time spent: 13.604/15.04 ms)

In step 2, maybe "the edgetype exists, we will not create" is more user-friendly than "Execution succeeded"