have unit-enabled test fail on non-existing unit

Closes #1708 (closed)

This MR ensures that the unit-enabled test in sylva-units fails when called on an undefined unit.

I could have made the choice of not failing in such a case, and return "not enabled", but this would have left room for typos leading to obscure errors.

What is changed:

• The code was wrongly returning true for an undefined unit, if units_enabled_default was true (case of management.values.yaml), because in that case it was returning units_enabled_default... this is fixed
• a fail with an error message is now done if no unit definition is found
• the rest is a code flow and indentation change without behavior change: we now first test the error case, and eventually fail, and in the normal case does we do not need to be in an if