Use 'user-error' in a few calendar files.

* lisp/calendar/appt.el (appt-add):
* lisp/calendar/calendar.el (calendar-absolute-from-gregorian)
(calendar-generate):
* lisp/calendar/diary-lib.el (diary-mail-entries, diary-cyclic):
Replace 'error' with 'user-error'.

lisp/calendar/appt.el

@@ -511,13 +511,13 @@ The default is `appt-message-warning-time'."
   (interactive "sTime (hh:mm[am/pm]): \nsMessage: \n\
 sMinutes before the appointment to start warning: ")
   (unless (string-match appt-time-regexp time)
-    (error "Unacceptable time-string"))
+    (user-error "Unacceptable time-string"))
   (and (stringp warntime)
        (setq warntime (unless (string-equal warntime "")
                         (string-to-number warntime))))
   (and warntime
        (not (integerp warntime))
-       (error "Argument WARNTIME must be an integer, or nil"))
+       (user-error "Argument WARNTIME must be an integer, or nil"))
   (or appt-timer (appt-activate))
   (let ((time-msg (list (list (appt-convert-time time))
                         (concat time " " msg) t)))

lisp/calendar/calendar.el

@@ -1194,7 +1194,7 @@ return negative results."
   (let ((year (calendar-extract-year date))
         offset-years)
     (cond ((zerop year)
-           (error "There was no year zero"))
+           (user-error "There was no year zero"))
           ((> year 0)
            (setq offset-years (1- year))
            (+ (calendar-day-number date) ; days this year
@@ -1379,7 +1379,7 @@ Optional integers MON and YR are used instead of today's date."
   ;; stands, almost all other calendar functions (eg holidays) would
   ;; at best have unpredictable results for such dates.
   (if (< (+ month (* 12 (1- year))) 2)
-      (error "Months before January, 1 AD cannot be displayed"))
+      (user-error "Months before January, 1 AD cannot be displayed"))
   (setq displayed-month month
         displayed-year year)
   (erase-buffer)

lisp/calendar/diary-lib.el

@@ -1197,7 +1197,7 @@ ensure that all relevant variables are set.
 "
   (interactive "P")
   (if (string-equal diary-mail-addr "")
-      (error "You must set `diary-mail-addr' to use this command")
+      (user-error "You must set `diary-mail-addr' to use this command")
     (let ((diary-display-function 'diary-fancy-display))
       (diary-list-entries (calendar-current-date) (or ndays diary-mail-days)))
     (compose-mail diary-mail-addr
@@ -1969,7 +1969,7 @@ and %s by the ordinal ending of that number (that is, `st', `nd',
 An optional parameter MARK specifies a face or single-character
 string to use when highlighting the day in the calendar."
   (or (> n 0)
-      (error "Day count must be positive"))
+      (user-error "Day count must be positive"))
   (let* ((diff (- (calendar-absolute-from-gregorian date)
                   (calendar-absolute-from-gregorian
                    (diary-make-date month day year))))