Silence "config warnings" in text mode, except for config ... commands

[cmaglie]

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• The PR follows
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Prints configuration warnings in text mode only if running one of the config ... commands.
Previously if there was an error in the configuration file, the warning was printed on all commands run from the cli.
In JSON mode the warning is always returned.

Also improve the error message from:

schema not defined for key 'asdasd'

to

Invalid value in configuration: schema not defined for key 'asdasd'

What is the current behavior?

$ arduino-cli version 
schema not defined for key 'asdasd'
arduino-cli  Version: 0.0.0-git Commit:  Date: 
$ arduino-cli version --json
{
  "Application": "arduino-cli",
  "Commit": "",
  "Date": "",
  "Status": "",
  "VersionString": "0.0.0-git",
  "warnings": [
    "schema not defined for key 'asdasd'"
  ]
}
$ arduino-cli config get directories.data
schema not defined for key 'asdasd'
/home/cmaglie/.arduino15

What is the new behavior?

$ arduino-cli version
arduino-cli  Version: 0.0.0-git Commit:  Date: 
$ arduino-cli version --json
{
  "Application": "arduino-cli",
  "Commit": "",
  "Date": "",
  "Status": "",
  "VersionString": "0.0.0-git",
  "warnings": [
    "Invalid value in configuration: schema not defined for key 'asdasd'"
  ]
}
$ arduino-cli config get directories.data
Invalid value in configuration: schema not defined for key 'asdasd'
/home/cmaglie/.arduino15

Does this PR introduce a breaking change, and is titled accordingly?

Other information

Partially fix #2638

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.28%. Comparing base (3f0d43f) to head (0beb1b6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2641      +/-   ##
==========================================
- Coverage   70.29%   70.28%   -0.02%     
==========================================
  Files         222      222              
  Lines       21308    21309       +1     
==========================================
- Hits        14978    14976       -2     
- Misses       5153     5155       +2     
- Partials     1177     1178       +1     

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[alessio-perugini]

Tested on ubuntu is working.

Is it intentional that when requesting the command below the json output omits the warnings key, but when requesting with a subcommand different from config the warnings key appears?

$ arduino-cli config get directories.data --json

"/home/ale/.arduino15"

$ arduino-cli version --format json
{
  "Application": "arduino-cli",
  "Commit": "",
  "Date": "",
  "Status": "",
  "VersionString": "0.0.0-git",
  "warnings": [
    "Invalid value in configuration: schema not defined for key 'asldfasdfasdf'"
  ]
}

[cmaglie]

Is it intentional that when requesting the command below the json output omits the warnings key, but when requesting with a subcommand different from config the warnings key appears?

Yes, that's because the returned JSON is a string and there is no way to fit a warning field inside.
We should have made it an object instead, like:

{
  "value": "/home/ale/.arduino15",
  "warnings": [
    "Invalid value in configuration: schema not defined for key 'asldfasdfasdf'"
  ]
}

but now it's too late, it would be for CLI 2.0.0 :-)