Use user-error instead of error where applicable

[luismbo]

Quoting from section 11.7.3.1 How to Signal an Error in the GNU Emacs Lisp Reference Manual (version 26.2):

"[user-error] behaves exactly like error, except that it uses the error symbol user-error rather than error. [It] is intended to report errors on the part of the user, rather than errors in the code itself. For example, if you try to use the command Info-history-back (l) to move back beyond the start of your Info browsing history, Emacs signals a user-error. Such errors do not cause entry to the debugger, even when debug-on-error is non-nil."