config: Make read_outputs failable

Previously we ignored when we had no output configuration
**and** failed to apply the automatically created one.

This leads to two problems:
- If this happens on startup, we end up with no outputs being added to the shell and we quit.
- If this happens later, we might end up in an inconsistent state, where the shell thinks we have an output, when it didn't light up for similar reasons.

Thus `read_outputs` is failable and handling that very much depends on
the where is was called from, because `read_outputs` doesn't know what
configuration was active before.

Thus make it failable and provide useful mitigations everywhere
possible:
- Try to enable just one output in case we fail on startup.
- Don't enable any additional outputs, when we fail on hotplug.
- Log the error like previously in any other case (and come up with more
  mitigations, once we understand these cases better).

src/backend/winit.rs

@@ -227,7 +227,7 @@ pub fn init_backend(
         .add_heads(std::iter::once(&output));
     {
         state.common.add_output(&output);
-        state.common.config.read_outputs(
+        if let Err(err) = state.common.config.read_outputs(
             &mut state.common.output_configuration_state,
             &mut state.backend,
             &state.common.shell,
@@ -236,7 +236,9 @@ pub fn init_backend(
             &state.common.xdg_activation_state,
             state.common.startup_done.clone(),
             &state.common.clock,
-        );
+        ) {
+            error!("Unrecoverable output config error: {}", err);
+        }
         state.common.refresh();
     }
     state.launch_xwayland(None);