Detect incomplete s commands

Check for commands of the form s/.../... (no final /) -- works with / only,
because general case matches too much ordinary text, e.g. "sopranos" is like
"s/pran/s". Alert the user to the syntax error, and prevent the incomplete s
command from replacing that user's last line.
Rename regex to update_line to more accurately reflect function

Also invert logic, to match the new name.