JSON format for exitcode could be simplified

[haxtibal]

The JSON exitcode attribute from a /v1/checker?command=foo response is a bit nasty to deserialize in statically typed languages, because its type varies in uncommon way:

• if a command can be executed, the real exit code is propagated like {"excitcode": 0}, i.e. integer
• if a command is not available for execution, we get {"excitcode": {}}, i.e. empty object

Integer has no empty form. A better representation would IMO be integer or null. This could be deserialized out of the box to e.g. Optional<int>.

[LordHepipud]

Thank you for the issue. Honestly the exit code should be 3 in case something fails, to always be able to return UNKNOWN.
I believe there is some error with handling errors properly.