fix(cli): a missing [nodes] directory refuses with a clean message

An existence pre-check on the [nodes] pointer target fires before the
crate pipeline: the refusal names the absent directory and keeps the
[nodes] context, instead of leaking cargo metadata's os-error wrap.
Existing-but-broken crates keep the ArtifactMissing/CargoMetadata
errors under the pointer prefix.

closes #245

crates/aura-cli/src/project.rs

@@ -59,6 +59,10 @@ pub enum ProjectError {
     Charter(String),
     MultiCrate(usize),
     NodesPointer { pointer: String, inner: Box<ProjectError> },
+    /// #245: the `[nodes]` pointer target does not exist — checked before
+    /// handing off to `load_crate`, so this never reaches the cargo-metadata
+    /// probe (whose raw os-error text would otherwise leak through).
+    PointerDirMissing(PathBuf),
 }
 
 impl fmt::Display for ProjectError {
@@ -95,6 +99,7 @@ impl fmt::Display for ProjectError {
                 f,
                 "node crate at `{pointer}` (from [nodes] in Aura.toml): {inner}"
             ),
+            Self::PointerDirMissing(p) => write!(f, "{} does not exist", p.display()),
         }
     }
 }
@@ -488,9 +493,19 @@ pub fn load(root: &Path, release: bool) -> Result<ProjectEnv, ProjectError> {
                 None
             }
         }
-        [pointer] => Some(load_crate(&root.join(pointer), release).map_err(|e| {
-            ProjectError::NodesPointer { pointer: pointer.clone(), inner: Box::new(e) }
-        })?),
+        [pointer] => {
+            let dir = root.join(pointer);
+            if !dir.is_dir() {
+                return Err(ProjectError::NodesPointer {
+                    pointer: pointer.clone(),
+                    inner: Box::new(ProjectError::PointerDirMissing(dir)),
+                });
+            }
+            Some(load_crate(&dir, release).map_err(|e| ProjectError::NodesPointer {
+                pointer: pointer.clone(),
+                inner: Box::new(e),
+            })?)
+        }
         many => return Err(ProjectError::MultiCrate(many.len())),
     };
     Ok(ProjectEnv {