Return exit code > 0 if some diffs were off

[tamtamchik]

Right now, even though differences are shown in the log, the diffyscan still exits with a 0 error code.
In case of an overwhelming amount of sources to check, it's easy to miss the error.

Example: got this in #55 while running full directory assertion command: diffyscan ./config_samples/bsc/testnet -y