Ensure an output_path is always present

Skip cases where this is not the case, as the original nixtract does

src/error.rs

@@ -2,8 +2,8 @@ pub type Result<T> = std::result::Result<T, Error>;
 
 #[derive(thiserror::Error, Debug)]
 pub enum Error {
-    #[error("Could not parse the Nix CLI output: {0}")]
-    SerdeJSON(#[from] serde_json::Error),
+    #[error("Could not parse the Nix CLI output for attribute path: {0}: {1}")]
+    SerdeJSON(String, serde_json::Error),
 
     #[error("Nix exited with a non-zero exit code: {0:#?}: {1}")]
     NixCommand(Option<i32>, String),

src/lib.rs

@@ -113,7 +113,7 @@ pub fn nixtract(
                 tx.clone(),
             ) {
                 Ok(_) => {}
-                Err(e) => log::error!("Error processing derivation: {}", e),
+                Err(e) => log::warn!("Error processing derivation: {}", e),
             }
         });
     });

src/nix/describe_derivation.rs

@@ -124,7 +124,10 @@ pub fn describe_derivation(
     }
 
     // Parse the stdout as JSON
-    let description: DerivationDescription = serde_json::from_str(stdout.trim())?;
+    let description: DerivationDescription = match serde_json::from_str(stdout.trim()) {
+        Ok(description) => description,
+        Err(e) => return Err(Error::SerdeJSON(attribute_path.to_owned(), e)),
+    };
 
     Ok(description)
 }

src/nix/find_attribute_paths.rs

@@ -75,7 +75,15 @@ pub fn find_attribute_paths(
             return Err(Error::NixCommand(output.status.code(), stderr.to_string()));
         } else {
             let attribute_paths: AttributePaths =
-                serde_json::from_str(line.trim_start_matches("trace: "))?;
+                match serde_json::from_str(line.trim_start_matches("trace: ")) {
+                    Ok(attribute_paths) => attribute_paths,
+                    Err(e) => {
+                        return Err(Error::SerdeJSON(
+                            attribute_path.clone().unwrap_or_default(),
+                            e,
+                        ))
+                    }
+                };
             res.push(attribute_paths);
         }
     }