[core] remove fail when default config file is missing

@barbalberto @lornat75 @drdanz this patch adds tests for following ResourceFinder scenarios:
 * user calls `rf.setDefaultConfigFile("config.ini")`, and that file is present (config should succeed)
 * user calls `rf.setDefaultConfigFile("config.ini")`, but that file is not present (config should succeed)
 * user specifies `--from config.ini`, and that file is present (config should succeed)
 * user specifies `--from config.ini`, but that file is not present (config should fail)

Is this the desired behavior?

It also adds a fix to make the tests pass. Specifically, it checks for `from` earlier, before defaults get loaded.