Dart error handling

[sneurlax]

Following up on #37:

The following additional functions have poor error-handing and could benefit from similar changes as found in #37:

• scanOutputs
• cancelTransaction
• recoverWallet
• initializeNewWallet
• createTransaction
• getChainHeight
• getAddressInfo
• deleteWallet
• openWallet
• txHttpSend

Fair but could be good:

• getTransactions
• validateSendAddress

Good but could be better:

• getWalletBalances
• getMnemonic

Lots of catches could use additional error context: where we return the original/passed error, add extra contextual information.