bug#10857: ucs-insert deals inconsistently with errors

[Juanma Barranquero <lekktu@gmail.com>]

Package: emacs
Severity: minor


`ucs-insert' does not deal very consistently with errors.

Two anomalies:

1)  M-x ucs-insert <RET> zzz <RET>   => "Not a Unicode character code: nil"
    Which is caused by `read-char-by-name' not having a way to pass
back what the user really typed. Still, I typed "zzz", not "nil", so
the message is unhelpful.

2) When called from lisp code, it deals differently with erroneous
strings and erroneous non-strings:
    (ucs-insert 'zzz)  =>  "Not a Unicode character code: zzz"   ;; correct
    (ucs-insert "zzz")  =>  any non-hex string is turned into ^@ and
inserted, and no error is produced.

The second problem can be trivially fixed with (not (string-match-p
"[^[:xdigit:]]" character)), though the docstring of `ucs-insert' does
not really say much about the valid forms the CHARACTER arg can take.

    Juanma