feat: Expose newFutureError for developer friendliness

[devlzcode]

Becuase the rpcclient.Response struct's fields are not exported we can not mirror the functionality of newFutureError ourselves. This is an issue especially when creating custom commands because we are forced to panic any errors in the *Async function. For example:

func SignRawTransactionWithKeyAsync(client *rpcclient.Client, tx *wire.MsgTx, privKeysWIF []string) FutureSignRawTransactionWithKeyResult {
    txHex := ""
    if tx != nil {
        buf := bytes.NewBuffer(make([]byte, 0, tx.SerializeSize()))
        if err := tx.Serialize(buf); err != nil {
            responseChan := make(chan *rpcclient.Response, 1)
            responseChan <- &rpcclient.Response{result: nil, err: err} // Can not compile because 'unknown field err in struct literal of type rpcclient.Response'
            return responseChan
        }
        txHex = hex.EncodeToString(buf.Bytes())
    }
    cmd := NewSignRawTransactionKeyCmd(txHex, &privKeysWIF)
    return client.SendCmd(cmd)
}

[coveralls]

Pull Request Test Coverage Report for Build 7663610047

Details

• -3 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.02%) to 56.765%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcclient/infrastructure.go	0	3	0.0%
<!--	Total:	0	3	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
connmgr/connmanager.go	3	86.27%
peer/peer.go	3	74.6%
<!--	Total:	6		-->

Totals
Change from base Build 7635501416:	0.02%
Covered Lines:	29215
Relevant Lines:	51467

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💛 - Coveralls