check funder balance is sufficient to fund all accounts before funding any

[zeroXbrock]

Motivation

• Another Spam Step Error #89
• better funding message #87

Users were seeing low-level error messages during the funding step which didn't explain the high-level problem. This change stops you before that can happen.

Solution

• sum up total cost of funding accounts in fund_accounts and error if the sender has less than that balance
  • takes into account gas price and adds 10% in case of future congestion
• shorten log to make it not seem like an error

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes