Improve validation when reading transitions config

[siladu]

When I got the key name or validatorselectionmode enum value wrong in the transitions config, my config was either ignored or I got incorrect behaviour at runtime.

An improvement would be to throw an error whenever an invalid key or enum value is attempted.

A further improvement would be to make a useful suggestion for the user, e.g. user types 'validatormode', respond with "Did you mean validatorselectionmode?" Or at least list enumerate the possible keys/values.

[antonydenyer]

In addition to this validation should be performed for when you specify validatorcontractaddress.

You can specify

"qbft": [{
  "block": 20,
  "validatorcontractaddress": "0x0000000000000000000000000000000000008888"
}]

without any warnings or errors. validatorselectionmode should be either required when specifying validatorcontractaddress or inferred from it's presence