Refund if sign() failed

[volovyks]

We need to refund the user's deposit if the sign() call fails for any reason. But let's ensure it is impossible to drain funds from our contract with bad requests.
Proposed by @gagdiez here.

[ailisp]

There are three cases to consider:

1. when the sign call fails (not the cross contract call invoked by sign), near will automatically return attached deposit back to the caller: https://docs.near.org/build/smart-contracts/anatomy/environment#attached-deposit
2. when the sign call succeed, we should refund the difference, this is the proposal by @gagdiez
3. when the sign call success, but the cross contract call invoked by sign, i.e., sign_helper, clear_state_on_finish, return_signature_on_finish fails, we should refund the remaining over required_deposit

[volovyks]

@ailisp

1. Ok
2. Agree
3. Why not all? It is not the user's fault that one of our functions failed.

[volovyks]

Blocked by https://github.com/near/transfer/issues/40

[volovyks]

We will push back with near/transfer#40 for now.