Improve error handling & fix clippy lints

[hannahfluch]

In this pull request, I first updated the source according to the clippy lint comparison_to_empty, which can be a lot faster than using the equality operator.

I also "fixed" the issue #29 with improved error handling. I was not able to fix the root cause, being an invalid utf-8 character. But before the program would just panic, whereas now, it prints the replacement character: U+FFFD instead.