scripts: kernel-doc: prevent a KeyError when checking output

scripts: kernel-doc: prevent a KeyError when checking output

If a file sent to KernelFiles.msg() method doesn't exist, instead
of producing a KeyError, output an error message.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/linux-doc/cover.1747719873.git.mchehab+huawei@kernel.org/T/#ma43ae9d8d0995b535cf5099e5381dace0410de04
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <4efa177f2157a7ec009cc197dfc2d87e6f32b165.1747817887.git.mchehab+huawei@kernel.org>