chg: [nextcloud] enhance error reporting upong relation failing

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

nextcloud/hooks/web_proxy-relation-joined

@@ -6,9 +6,16 @@ DOMAIN=$(relation-get domain) || exit 1
 URL="$(relation-get url)" || exit 1
 PROTO="${URL%%://*}"
 
-trusted_domains="$(
+if ! trusted_domains="$(
     compose -q --no-relations --no-init occ "$MASTER_BASE_SERVICE_NAME" \
-         config:system:get trusted_domains)"
+         config:system:get trusted_domains)"; then
+    err "Couldn't get 'trusted_domains'. Here's the ouput:"
+    echo "$trusted_domains" | prefix "  | " >&2
+
+    echo "If the code of nextcloud is already there (command occ is found), but " >&2
+    echo "the database is not yet created, this situation will arise." >&2
+    exit 1
+fi
 
 occ_opts=(
     ## necessary as nextcloud do not detect correctly those, and behind