fix(http): respond with 503 when inverter unavailable at startup

[brandon1024]

OpenEVT does not have any persistance of any kind. In some situations, whenever the application is restarted for any reason (e.g. upgrades), services that scrape the inverter status can get zero values briefly until OpenEVT has time to talk to the inverter. This can lead to some pretty nasty visualization behaviour (big spikes followed by big corrections).

This revision adjusts the web server to respond with a 503 when the status is not yet available, as opposed to returning zeroes. Services that scrape the endpoint should be able to use this to handle the error accordingly, skipping the datapoints rather than plotting zeroes.

Additionally, bump Go to latest release and upgrade dependencies.

[brandon1024]

Here's a quick demonstration of the problem. When we restart the container, we briefly get a zero struct:

Error: no container with name or ID "home-assistant-prod" found: no such container
ERRO[0000] User-selected graph driver "vfs" overwritten by graph driver "overlay" from database - delete libpod local files to resolve. 
 May prevent use of images created by other tools 
{
  "InverterId": "",
  "Module1": {
    "ModuleId": "",
    "FirmwareVersion": "",
    "InputVoltageDC": 0,
    "OutputPowerAC": 0,
    "TotalEnergy": 0,
    "Temperature": 0,
    "OutputVoltageAC": 0,
    "OutputFrequencyAC": 0
  },
  "Module2": {
    "ModuleId": "",
    "FirmwareVersion": "",
    "InputVoltageDC": 0,
    "OutputPowerAC": 0,
    "TotalEnergy": 0,
    "Temperature": 0,
    "OutputVoltageAC": 0,
    "OutputFrequencyAC": 0
  }
}
ERRO[0000] User-selected graph driver "vfs" overwritten by graph driver "overlay" from database - delete libpod local files to resolve. 
 May prevent use of images created by other tools 
{
  "InverterId": "30582932",
  "Module1": {
    "ModuleId": "30582932",
    "FirmwareVersion": "112/121",
    "InputVoltageDC": 34.7734375,
    "OutputPowerAC": 31.609375,
    "TotalEnergy": 194.08056640625,
    "Temperature": 22.1953125,
    "OutputVoltageAC": 235.0625,
    "OutputFrequencyAC": 50.05078125
  },
  "Module2": {
    "ModuleId": "30582933",
    "FirmwareVersion": "112/121",
    "InputVoltageDC": 34.7734375,
    "OutputPowerAC": 26.84375,
    "TotalEnergy": 165.9315185546875,
    "Temperature": 22.59375,
    "OutputVoltageAC": 235.0625,
    "OutputFrequencyAC": 50.05078125
  }
}