Give suggestion to use `--replace-missing-char` in the error message?

[user202729]

I think this would help the user to discover the option. Currently the error message just say "PDF contains a unicode char that does not exist in the font", maybe add "or pass --replace-missing-char=" " to forcefully ignore this error" there.

Alternatively, it's also possible to replace it with some dummy character, then if the extracted title does not contain any dummy character then silently ignore the error, since there's no harm.

Making replace-missing-char the default is also an option, but I think it's a conscious design choice to not make it so. In my case, can be I'd overwhelmingly want to use that option however.