Improve the command for set memo flag

[HaoyangLiu]

bnbcli account_flags enable-memo-checker

bnbcli account_flags disable-memo-checker

We can use the above two commands to conveniently set the memo checker related account flag. However, if the memo flag is already set to what it is expected to be, calling the above commands will waste much transaction fee. The transaction fee is 1BNB which is very expensive.
So it would be better if the above commands can defect this duplicated account flags setting and just return error.