core: return more meaningful error when no device is found for activation

Example:
 $ nmcli con add type eth con-name profile ifname nosuch
 $ nmcli con up profile
or
 # ip link set eth0 down
 $ nmcli con add type eth con-name my-eth0 ifname eth0
 $ nmcli con up my-eth0

Related to: 6fc3736c7a

(cherry picked from commit 02989896af)

src/nm-manager.c

@@ -3053,7 +3053,7 @@ validate_activation_request (NMManager *self,
 			g_set_error_literal (error,
 			                     NM_MANAGER_ERROR,
 			                     NM_MANAGER_ERROR_UNKNOWN_DEVICE,
-			                     "This connection requires an existing device.");
+			                     "No suitable device found for this connection.");
 			goto error;
 		}