Make read-char-by-name signal an error for invalid input.

Make read-char-by-name signal an error for invalid input.

* international/mule-cmds.el (read-char-by-name): Signal an error
if the user does not supply a valid character.

* editfns.c (Finsert_char): Since read-char-by-name now signals an
error for invalid chars, don't check for a nil return value.