Update: add a space after "combining" characters and catch a few more invalid UTF...

Update: add a space after "combining" characters and catch a few more invalid UTF-8 sequences

Previously, I just printed a space instead of printing the "combining" characters.
It occurred to me that I could print a space following a known "combining" character to cause it to combine into a space.
This makes things easier to view and still displays the combining character instead of hiding it behind a blank space.
The downside is that this might cause problems if someone tried to copy and paste these combined characters.

Catch a few more invalid UTF-8 sequences that I came across while making these changes.
Fix an existing invalid UTF-8 sequence detection that seems to have been incomplete and incorrect.