File collector: better error message when path does not exist

[ghost]

If the file collector is configured to read data from a non-existing directory, the error message is:

intelmq.lib.exceptions.InvalidArgument: Argument 'path' is invalid. Should be of type: directory. Got '/tmp/foobar/'.

The error message should say, that the directory does not exist.

Related to the parameter refactoring -> version 3.0