cmd: Adjust config load logs/errors

[francislavoie]

Sometimes, users use Caddyfile file names that don't match our adapter-guessing rules, and this causes the config to be loaded as JSON. The error message is very confusing when that happens. I made some adjustments to improve it.

Before:

2024/01/10 19:54:15.805	INFO	using provided configuration	{"config_file": "caddyfile.test", "config_adapter": ""}
Error: loading initial config: decoding request body: invalid character 'd' looking for beginning of object key string

After:

2024/01/10 19:54:32.815	INFO	using config from file	{"file": "caddyfile.test"}
Error: config is not valid JSON: invalid character 'd' looking for beginning of object key string; did you mean to use '--adapter'?

Now, we're quickly doing a json.Unmarshal just to test that the JSON is valid (there is json.Validate() but it only returns a bool and not errors) before returning it to the caller after loading the config file.

Also, I moved the adapter log to be after the adapter guessing logic so that it doesn't show up empty in a confusing way in the logs. I also split the stdin and file log messages for clarity.

I'm not sure about the last bit though ; did you mean to use '--adapter'? is there a better way we could word that without being ambiguous? 🤔 The problems is "what if they actually meant to load JSON" then a mention like this might be confusing for those users. So it's hard to go too far in either direction with the warning.

[mholt]

LGTM -- what about this error message?