Marginally refactor function to add SSH keys to current agent

Does this improve readability?

site-lisp/db-utils.el

@@ -840,16 +840,19 @@ This is `db-light' and `solarized-light'."
   "Synchronously add key in KEY-FILE to currently running ssh-agent.
 
 PASSWORD-FN is supposed to be a function returning the password
-for KEY-FILE; PASSWORD-FN is called on demand.
+for KEY-FILE; PASSWORD-FN is called on demand.  If KEY-FILE is
+not readable, this function errors out.
 
 This function uses ssh-add to add the key to the currently
 running ssh-agent and waits for the process to finish."
+  (let ((key-file (expand-file-name key-file)))
+
+    (unless (file-readable-p key-file)
+      (user-error "SSH key %s does not exist, aborting" key-file))
+
     (with-environment-variables (("SSH_ASKPASS_REQUIRE" "never"))
-    (let* ((key-file (expand-file-name key-file))
-           (proc (make-process :name "ssh-add"
-                               :buffer nil
-                               :command (list "ssh-add" key-file)
-                               :filter #'(lambda (process output)
+
+      (let* ((ssh-add-handle-output #'(lambda (process output)
                                         (cond
                                           ((string= (format "Enter passphrase for %s: "
                                                             key-file)
@@ -862,20 +865,28 @@ running ssh-agent and waits for the process to finish."
                                                (string= output "\n"))
                                            ;; Ignore harmless output
                                            t)
-                                             (t (message "Unknown output received from ssh-agent: %s" output))))
-                               :sentinel #'(lambda (_ event)
+                                          (t (message "Unknown output received from ssh-agent: %s" output)))))
+
+             (ssh-add-handle-event-change #'(lambda (_ event)
                                               (cond
                                                 ((string= event "finished\n")
                                                  (message "Successfully added %s to local SSH agent"
                                                           key-file))
                                                 (t (message "Adding SSH key %s failed, ssh-add process reached state %s"
                                                             key-file
-                                                           event)))))))
+                                                            event)))))
+
+             (proc (make-process :name "ssh-add"
+                                 :buffer nil
+                                 :command (list "ssh-add" key-file)
+                                 :filter ssh-add-handle-output
+                                 :sentinel ssh-add-handle-event-change)))
+
         ;; We are waiting for the process to finish, to not let its output
         ;; intermingle with others.  XXX: is there a more standard way to wait for
         ;; a process to finish?
         (while (process-live-p proc)
-        (sit-for 0.2)))))
+          (sit-for 0.2))))))
 
 (defun db/ssh-key-hash-from-filename (key-file)
   "Return the SHA256 hash value of the SSH key located in KEY-FILE.