Changes the accounts_representatives RPC to return per account response

[thsfs]

The previous implementation returned only an error result in case any of the provided accounts were invalid or not found. This implementation handles each account separately returning either a successful or an error response.

Example:

    "representatives": {
        "nano_3wfddg7a1paogrcwi3yhwnaerboukbr7rs3z3ino5toyq3yyhimo6f6egij6": "nano_3wfddg7a1paogrcwi3yhwnaerboukbr7rs3z3ino5toyq3yyhimo6f6egij6",
        "nano_36uccgpjzhjsdbj44wm1y5hyz8gefx3wjpp1jircxt84nopxkxti5bzq1rnz": "error: Bad account number",
        "nano_1hrts7hcoozxccnffoq9hqhngnn9jz783usapejm57ejtqcyz9dpso1bibuy": "error: Account not found"
    }

Reference issue: #3752