Clarify import cert command
Clarify import cert command

Add hint that the certificate to be imported must be in PEM format (CRT
does not parse).

Improved error message to mention that the given path must be accessible
by the web server user.

Signed-off-by: Vincent Petry <87e1f221a672a14a323e57bb65eaea19d3ed3804@nextcloud.com>

andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,nextcloud/server