The opengov tips end up with empty proposal (no preimage) if no funds

[mordamax]

The problem appeared in paritytech/cumulus#2294
The proposal was submitted but without a preimage, which led to no actions, once it's executed
Reason was that the bot account had no enough funds, so the wasn't noted
I reproduced it locally, leaving bot with no funds it goes without any errors straight to adding a proposal without preimage

the solution would be

• explicitly checking if the preimage exists on chain before creating a proposal. If no preimage - just fail and notify in PR comments as usual fix: #60, Check if preimage exists on chain before creating a proposal #61
• top up account with more funds