org-babel-eval: Return command output even upon failure

* lisp/ob-eval.el (org-babel-eval): Return the available command
output even when the command fails.

This is useful for bash scripts that can produce useful output even
upon failing.  The error window is still displayed to inform users.

Reported-by: Rudolf Adamkovič <salutis@me.com>
Link: https://orgmode.org/list/m2czakgpk8.fsf@me.com

lisp/ob-eval.el

@@ -71,7 +71,8 @@ its results, otherwise display STDERR with
 		    (compilation-mode))
 		  ;; Compilation-mode enforces read-only, but Babel expects the buffer modifiable.
 		  (setq buffer-read-only nil))))
-	    nil)
+            ;; Return output, if any.
+	    (buffer-string))
 	(buffer-string)))))
 
 (defun org-babel-eval-read-file (file)