install: Expand error message for missing SSL cert slightly.

It wasn't obvious reading this message that you can perfectly well
bring your own SSL/TLS certificate; unless you read quite a bit
between the lines where we say "could not find", or followed the link
to the detailed docs, the message sounded like you had to either use
--certbot or --self-signed-cert.

So, explicitly mention the BYO option.  Because the "complete chain"
requirement is a bit tricky, don't try to give instructions for it
in this message; just refer the reader to the docs.

Also, drop the logic to identify which of the files is missing; it
certainly makes the code more complex, and I think even the error
message is actually clearer when it just gives the complete list of
required files -- it's much more likely that the reader doesn't know
what's required than that they do and have missed one, and even then
it's easy for them to look for themselves.

scripts/lib/install

@@ -112,15 +112,20 @@ fi
 # Check early for missing SSL certificates
 if [ "$PUPPET_CLASSES" = "zulip::voyager" ] && [ -z "$USE_CERTBOT""$SELF_SIGNED_CERT" ] && { ! [ -e "/etc/ssl/private/zulip.key" ] || ! [ -e "/etc/ssl/certs/zulip.combined-chain.crt" ]; }; then
     set +x
-    echo
-    echo "Could not find SSL certificates!"
-    for f in "/etc/ssl/private/zulip.key" "/etc/ssl/certs/zulip.combined-chain.crt"; do
-        [ -e "$f" ] || echo " - $f is missing!"
-    done
     cat <<EOF
 
-See https://zulip.readthedocs.io/en/latest/production/ssl-certificates.html for help.
+No SSL certificate found.  One or both required files is missing:
-For non-production testing, try the --self-signed-cert option.
+    /etc/ssl/private/zulip.key
+    /etc/ssl/certs/zulip.combined-chain.crt
+
+Suggested solutions:
+ * For most sites, the --certbot option is recommended.
+ * If you have your own key and cert, see docs linked below
+   for how to install them.
+ * For non-production testing, try the --self-signed-cert option.
+
+For help and more details, see our SSL documentation:
+  https://zulip.readthedocs.io/en/latest/production/ssl-certificates.html
 
 Once fixed, just rerun scripts/setup/install; it'll pick up from here!