File unlock errors now issue warnings (Bug#46397)

File unlock errors now issue warnings (Bug#46397)

The primary idea is to allow `kill-buffer' and `kill-emacs' to
complete even if Emacs has trouble unlocking the buffer's file.

* lisp/userlock.el (userlock--handle-unlock-error): New function, call
`display-error'.
* src/filelock.c (unlock_file_body): New function, do what
'unlock_file' used to.
(unlock_file_handle_error): New function, call
`userlock--handle-unlock-error' with the captured error.
(unlock_file): Handle `file-error' conditions by calling the handler
defined above.
* test/src/filelock-tests.el (filelock-tests-kill-buffer-spoiled):
(filelock-tests-unlock-spoiled): Modify to test new behavior.