Quality improvements for accounts_balances RPC

[thsfs]

Address issue reported by nanocurrency/nano-docs#665. Related to #4223, but this applies to accounts_balances RPC.

accounts_balances

Request:

{  
  "action": "accounts_balances",  
  "accounts": ["nano_3t6k35gi95xu6tergt6p69ck76ogmitsa8mnijtpxm9fkcm736xtoncuohr3", "nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz", "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy"]  
}

Response (note the second account number is a not found account -- it returns a 0 balance):

{
    "balances": {
        "nano_3t6k35gi95xu6tergt6p69ck76ogmitsa8mnijtpxm9fkcm736xtoncuohr3": {
            "balance": "325586539664609129644855132177",
            "pending": "2309370929000000000000000000000000",
            "receivable": "2309370929000000000000000000000000"
        },
        "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy": {
            "balance": "0",
            "pending": "0",
            "receivable": "0"
        }
    },
    "errors": {
        "nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz": "Bad account number"
    }
}

Request & Response when there are no errors:

Request:

{  
  "action": "accounts_frontiers",  
  "accounts": ["nano_3t6k35gi95xu6tergt6p69ck76ogmitsa8mnijtpxm9fkcm736xtoncuohr3"]  
}

Response:

{
    "balances": {
        "nano_3t6k35gi95xu6tergt6p69ck76ogmitsa8mnijtpxm9fkcm736xtoncuohr3": {
            "balance": "325586539664609129644855132177",
            "pending": "2309370929000000000000000000000000",
            "receivable": "2309370929000000000000000000000000"
        }
    }
}

Request & Response when all entries return errors:

Request:

{  
  "action": "accounts_frontiers",  
  "accounts": ["nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz"]  
}

Response:

{
    "errors": {
        "nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz": "Bad account number"
    }
}