Improve error message when run from a buffer that doesn't visit a file

[phst]

Summary

In this case, users currently get a confusing type error. Make the error message more obvious.

[klimek]

lg

include-fixer/tool/clang-include-fixer.el

(clang-include-fixer--start #'clang-include-fixer--add-header
                               (format "-query-symbol=%s" symbol)))
 
 (defun clang-include-fixer--start (callback &rest args)
   "Asynchronously start clang-include-fixer with parameters ARGS.
 The current file name is passed after ARGS as last argument.  If
 the call was successful the returned result is stored in a
 temporary buffer, and CALLBACK is called with the temporary
 buffer as only argument."
+  (unless buffer-file-name
+    (user-error "clang-include-fixer works only in buffers that visit a file"))
   (let ((process (if (fboundp 'make-process)
                      ;; Prefer using ‘make-process’ if available, because
                      ;; ‘start-process’ doesn’t allow us to separate the
                      ;; standard error from the output.
                      (clang-include-fixer--make-process callback args)
                    (clang-include-fixer--start-process callback args))))
     (save-restriction
       (widen)