Improve error messages for developers using FCL

[nialexsan]

Parse cadence errors into a json structure

[jribbink]

#1893 has just been merged, which is probably a good baby step. There's definitely more that can be done here (just wanted to drop a few of my ideas in case they are helpful)

• TransactionError is only surfaced for fcl.tx().once predicates (i.e. onceSealed). We could add this to the txStatus in .subscribe potentially?
• We don't have any well parsing in place for script errors. Ideally there would be a ScriptError type, maybe. And both this and TransactionError could maybe inherit from a common type (e.g. ExecutionError)
• There's no parsing beyond the error code and no formatting of the message. It's just surfaced easier to devs mostly. This was intentionally done because we want to be cautious here. If we try to parse too much, we could end up with broken error parsing later down the line if flow-go changes. We should do everything in our power to avoid this (people update FCL VERY infrequently, and broken versions could end up polluting the dApp ecosystem).... that being said... if there's a future-proof way to do this, I'm all ears.