fixed unwrap() error on invalid utf8

[alexanderkjall]

fixes #2

[wfxr]

Thanks for your contribution @alexanderkjall .

We can improve the error message but should not ignore the encoding error. csview should exit with a non-zero code since it only supports utf-8.

[alexanderkjall]

That sounds like a better solution, I just needed something like this to continue my fuzzing, feel free to close this PR.

For your information I also found a bug in an underlying library: phsym/prettytable-rs#130