[patch] minor patch for register.el

[Drew Adams]

If the attached patch is OK I will send a change log entry.

The patch does this:

1. Adds functions `register-insertable-p', `register-jumpable-p', and
`register-printable-p'.

2. In `jump-to-register':
   . Use `register-jumpable-p'.
   . Raise error if no such register or cannot jump to it.

3. In `view-register': Raise error if no such register.

4. In `list-registers': Raise error if `register-alist' is empty.

5. In `describe-register-1':
   . Use `register-printable-p'.
   . Wrap names with `...'.

6. In `insert-register':
   . Use `register-insertable-p'.
   . Raise error if no such register or cannot insert contents.

7. In `append-to-register' and `prepend-to-register':
   Name the register in error msg.

register-2013-02-19.patch
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