From: Harald Nordgren <haraldnordgren@gmail.com>

Explain the required Subject: prefix when a message file has no subject.
Terminate the error with a newline so Perl does not append its internal
source location.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.2375.git.git.1786267394375.gitgitgadget@gmail.com