Motivation
----------
The way how I fixed this is that both `Stdout` and `Stderr` are responded back to the client.
I don't think it's good practice to discard `$stderr` on success and to discard `$stdout` on error.
Let me know what you think. I'm still very new to Golang.
How to test
-----------
1. First of all you must be able to reproduce "CAPTCHA proof required" error (I guess you need to send a lot of messages to the same number)
2. Execute:
```
curl -X POST -H "Content-Type: application/json" 'http://localhost:8080/v2/send' \
-d '{"message": "Test via Signal API!", "number": "<SENDER_PHONE_NUMBER>", "recipients": [ "<RECIPIENT_PHONE_NUMBER>" ]}
'
```
3. See in the JSON response:
```
{"error":"Failed to send (some) messages:\n+49176xxxxxxxx: CAPTCHA proof required for sending to \"+49176xxxxxxxx\", available options \"RECAPTCHA, PUSH_CHALLENGE\" with challenge token \"1f209ee0-d487-4efc-xxxx-xxxxxxxxxxxx\", or wait \"86400\" seconds.\nTo get the captcha token, go to https://signalcaptchas.org/challenge/generate.html\nCheck the developer tools (F12) console for a failed redirect to signalcaptcha://\nEverything after signalcaptcha:// is the captcha token.\nUse the following command to submit the captcha token:\nsignal-cli submitRateLimitChallenge --challenge CHALLENGE_TOKEN --captcha CAPTCHA_TOKEN\nxxxxxxxxxxxxx\nFailed to send message\n"}
```
fix#403
