Connector export: create connector folder structure

This commit adds code so that the folder structure for every exported
connector is created. This folder structure is:

  output_path/neuron_id/[presynaptic|postsynaptic]/connector_id

In case a connector is not linked with the presynaptic_to or the
postsynaptic_to relation of the current project, a folder named
'unknown_<relation_id>' is created instead.
Cropping: make it possible to disable message creation

Not all modules that are using the cropping functionality want to have a message
created.
Cropping: make message creation code more readable
Cropping: add infrastructure to add more tile source types
Cropping: add support for tile source type 4
Cropping: return error from main handler if one happened
Connector export: use more precise naming