Alert the user when undefined input characters are encountered

[dkager]

• Return error when encountering undefined input character liblouis#11
• Abort processing on liblouis error.
• Show the phrase that caused the error.

[bertfrees]

Would leaving a warning message be enough?

[dkager]

Would leaving a warning message be enough?

If processing can be aborted on this warning, sure. But even if not it would be a good start. If some of the false negatives are cleared so the error log becomes shorter I think it's not a problem to scroll through it after conversion.

[bertfrees]

@dkager Maybe this is something you can look into when you have time?