warn instead of error if zero package catalogers are select - user might still run file metadata cataloger, for example

[tomersein]

What would you like to be added:
I am trying to scan but don't use the packages tasks. is it possible to disable it and use only files tasks?
Why is this needed:
want to separate tasks
Additional context:

[kzantow]

@tomersein you can disable all the package catalogers using the --select-catalogers -image,-directory, however it is currently an error if no catalogers are selected. We could probably change that to a warning so the remaining catalogers run.

[tomersein]

can be good, i might have time to implement it