org-capture: Demote error when capturing `file+headline' in non-Org buffers

* lisp/org-capture.el (org-capture-set-target-location): Warn when
  capturing a `file+headline' target and buffer is not in Org mode.

Fixes: 22352

lisp/org-capture.el

@@ -1,6 +1,6 @@
 ;;; org-capture.el --- Fast note taking in Org       -*- lexical-binding: t; -*-
 
-;; Copyright (C) 2010-2017 Free Software Foundation, Inc.
+;; Copyright (C) 2010-2018 Free Software Foundation, Inc.
 
 ;; Author: Carsten Dominik <carsten at orgmode dot org>
 ;; Keywords: outlines, hypermedia, calendar, wp
@@ -927,18 +927,24 @@ Store them in the capture property list."
 	   (_ (error "Cannot find target ID \"%s\"" id))))
 	(`(file+headline ,path ,headline)
 	 (set-buffer (org-capture-target-buffer path))
+	 ;; Org expects the target file to be in Org mode, otherwise
+	 ;; it throws an error.  However, the default notes files
+	 ;; should work out of the box.  In this case, we switch it to
+	 ;; Org mode.
 	 (unless (derived-mode-p 'org-mode)
-	   (error "Target buffer \"%s\" for file+headline not in Org mode"
-		  (current-buffer)))
+	   (org-display-warning
+	    (format "Capture requirement: switching buffer %S to Org mode"
+		    (current-buffer)))
+	   (org-mode))
 	 (org-capture-put-target-region-and-position)
 	 (widen)
 	 (goto-char (point-min))
 	 (if (re-search-forward (format org-complex-heading-regexp-format
 					(regexp-quote headline))
 				nil t)
-	     (goto-char (line-beginning-position))
+	     (beginning-of-line)
 	   (goto-char (point-max))
-	   (or (bolp) (insert "\n"))
+	   (unless (bolp) (insert "\n"))
 	   (insert "* " headline "\n")
 	   (beginning-of-line 0)))
 	(`(file+olp ,path . ,outline-path)