Add tests for config and Add exceptions to config

Add exceptions to config and raise them when
invalid or empty furious.yaml file is found. Change
config loader to enable it to work with a mock file.
Change module_import to raise an exception when
it is not found.
Tests cover importing of
	- existing and non-existing modules;
	- finding a yaml file that exist and does not exist
	- loading a valid yaml file
	- loading an invalid yaml file
	- loading the configured persistence module
        - loading a persistence module by invalid name