Fix JSON output issues

[Klaim]

This fixes several issues:

• potential unhandled exception thrown when a mix of args ends up calling json_up() but the json is empty;
• replaced explicit calls to json printing to the output by automatic printing to the output at the end of the program (after main()) - this is mainly to prevent the json output to end up invalid if called more than once:

{ // output by the first part of the operation
    "success": true,
}
{
   // some other values from the rest of the operation
}

This is invalid json because there is more than one root objects and makes it difficult or impossible to check the json ouput in tests.

• refactored a bit and added a few checks;
• added a way to cancel the print of json log from libmamba, used in mamba which already output the right json;

Note that these issues were spotted while making tests for #1577 , they are hard to reproduce otherwise.

[wolfv]

Looks great!

[wolfv]

For the linting you can just run pre-commit run --all.