Add error message for STEEM_UPVOTE_LOCKOUT_HF17

[Dzivenu]

On Steemit.com when trying to upvote a post within the last 12 hours prior to cashout, you get the error:

RPCError: Assert Exception:_db.head_block_time() < comment.cashout_time - STEEM_UPVOTE_LOCKOUT_HF17: Cannot increase payout within the last twelve hours before payout

However, when posting with SC2, you do not get any error message at all. All you get is a 500 error. Adding a concise error message will help devs deliver the message to users who encounter the app.

Also, it will save devs time they would have spent searching for the error, in the case where they wrongly assume the problem is from their app.

[bonustrack]

You should be able to see the error from the Steemd node in SteemConnect posting API response. What code are you using to display error?