nf-core lint generates an ERROR if modules.json is missing but doesn't exit

[drpatelh]

As you can see in this CI test for the fetchngs pipeline nf-core lint prints an ERROR at the top of the screen because modules.json isn't present but doesn't generate a system exit. I am not sure there is a need for the ERROR to be printed there because this is already reported in the lint results?

Also, I think we need to allow cases where modules.json isn't present because for example, fetchngs consists entirely of local modules at the moment.