fix: error message when trying to migrate a migrated config

[ccoVeille]

The check was done in the wrong place, so it was not triggered
when the config was already migrated.

Another error message was reported about jsonschema validation issues.
Because a v2 config file does not validate against the v1 schema.

That was misleading.

Before

$ golangci-lint migrate 
jsonschema: "linters.exclusions" does not validate with "/properties/linters/properties/exclusions/additionalProperties": additional properties 'presets' not allowed
jsonschema: "linters" does not validate with "/properties/linters/additionalProperties": additional properties 'default', 'settings' not allowed
jsonschema: "" does not validate with "/additionalProperties": additional properties 'version', 'formatters' not allowed
Failed executing command with error: the configuration contains invalid elements

After

$ golangci-lint migrate
The command is terminated due to an error: configuration version is already set: 2
exit status 3

[CLAassistant]

All committers have signed the CLA.

[ldez]

LGTM