Geary.Util.ConnectivityManager: Fix a spurious connectivity error report

When a general error occurs checking connectivity of a remote server,
only treat it as an error if GNetworkMonitor indicates the network is
available (i.e. that a default route exists). If there is no network,
it's not surprising that the error occurred.

This fixes a spurious error when the delayed check runs but the host
is offline.

src/engine/util/util-connectivity-manager.vala

@@ -166,7 +166,7 @@ public class Geary.ConnectivityManager : BaseObject {
                 // needlessly hassle the user with expected error
                 // messages.
                 GLib.NetworkConnectivity connectivity = this.monitor.connectivity;
-                if (connectivity == FULL ||
+                if ((this.monitor.network_available && connectivity == FULL) ||
                     (connectivity == LOCAL && is_local_address())) {
                     debug("Error checking %s [%s] reachable, treating unreachable: %s",
                           endpoint, connectivity.to_string(), err.message);