Fix (find-file "/dev/null")

Fix (find-file "/dev/null")

* lisp/files.el (find-file-noselect-1): When the file exists,
propagate any errors signaled by insert-file-contents,
instead of propagating them only when the file is unreadable.
This way, (find-file "/dev/null") gives a sensible diagnostic
"not a regular file" instead of the nonsense
"Maximum buffer size exceeded".

(cherry picked from commit b911029f96b14a03398198d5a849d6f4d0d1e071)