config referencing nonexistent CDF source leads to uncaught exception

[tarheel]

Example:

2019-07-06 22:33:50 PDT INFO: Successfully loaded contest config: /Users/leisenberg/code/rcv/src/test/resources/network/brightspots/rcv/test_data/test_set_treat_blank_as_undeclared_write_in/test_set_treat_blank_as_undeclared_write_in_config.json
2019-07-06 22:33:50 PDT SEVERE: Error opening file: /Users/leisenberg/code/rcv/src/test/resources/network/brightspots/rcv/test_data/test_set_treat_blank_as_undeclared_write_in/test_set_0_skipped_first_choice_cvr.json
java.io.FileNotFoundException: /Users/leisenberg/code/rcv/src/test/resources/network/brightspots/rcv/test_data/test_set_treat_blank_as_undeclared_write_in/test_set_0_skipped_first_choice_cvr.json (No such file or directory)
Check file path and permissions and make sure they are correct!
2019-07-06 22:33:50 PDT SEVERE: Error parsing candidate data: java.lang.NullPointerException

I guess it basically handles it fine, but maybe we should clean up the log messages a bit.

[tarheel]

At some point we updated the validation logic to complain if a source file is nonexistent, so there's nothing left to do here.