diagnostics: meaningful error when graph node has wrong number of inp…

…uts (bevyengine#4924)

# Objective

Currently, providing the wrong number of inputs to a render graph node triggers this assertion:

```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', /[redacted]/bevy/crates/bevy_render/src/renderer/graph_runner.rs:164:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

This does not provide the user any context.

## Solution

Add a new `RenderGraphRunnerError` variant to handle this case. The new message looks like this:

```
ERROR bevy_render::renderer: Error running render graph:
ERROR bevy_render::renderer: > node (name: 'Some("outline_pass")') has 2 input slots, but was provided 1 values
```

---

## Changelog

### Changed

`RenderGraphRunnerError` now has a new variant, `MismatchedInputCount`.

## Migration Guide

Exhaustive matches on `RenderGraphRunnerError` will need to add a branch to handle the new `MismatchedInputCount` variant.

crates/bevy_render/src/renderer/graph_runner.rs

@@ -41,6 +41,14 @@ pub enum RenderGraphRunnerError {
         expected: SlotType,
         actual: SlotType,
     },
+    #[error(
+        "node (name: '{node_name:?}') has {slot_count} input slots, but was provided {value_count} values"
+    )]
+    MismatchedInputCount {
+        node_name: Option<Cow<'static, str>>,
+        slot_count: usize,
+        value_count: usize,
+    },
 }
 
 impl RenderGraphRunner {
@@ -161,7 +169,13 @@ impl RenderGraphRunner {
                 .map(|(_, value)| value)
                 .collect();
 
-            assert_eq!(inputs.len(), node_state.input_slots.len());
+            if inputs.len() != node_state.input_slots.len() {
+                return Err(RenderGraphRunnerError::MismatchedInputCount {
+                    node_name: node_state.name.clone(),
+                    slot_count: node_state.input_slots.len(),
+                    value_count: inputs.len(),
+                });
+            }
 
             let mut outputs: SmallVec<[Option<SlotValue>; 4]> =
                 smallvec![None; node_state.output_slots.len()];