feat(errors): preserve jj-emitted errors when loading config

tests/error_tests.rs

@@ -20,7 +20,9 @@ fn test_all_error_variants() {
     };
     let _repo_locked = Error::RepositoryLocked;
     let _failed_dir = Error::FailedGettingCurrentDir;
-    let _failed_config = Error::FailedReadingConfig;
+    let _failed_config = Error::FailedReadingConfig {
+        context: "test".to_string(),
+    };
 
     // Application errors
     let cancelled = Error::Cancelled;