Respect disabled node coverage when a rule raises

[chupakobra6]

Related to #10.

When a rule raises, add_error_result() calls append_nodes_dict() even when node coverage is disabled. This can traverse a large IDS and add coverage to the report despite running without -nc / --node-coverage.

Apply the same track_node_dict guard already used by assert_(). Preserve the failed result and its exception/traceback. With coverage enabled, errors still register the filled nodes as uncovered.

Validation:

• Six regression cases cover errors before/after an assertion, cross-IDS context, nonzero occurrences, coverage on/off, and real netCDF validation. They check the failed result, traceback, XML/text failure counts, and coverage output. With coverage disabled, the unit tests explicitly forbid filled-node traversal. Three cases fail on unchanged develop; all pass with this change.
• Full suite: 191 passed, 2 existing skips on macOS/Python 3.12 with IMAS-Python 2.3.0, and on Python 3.10 with minimum IMAS-Python 2.2.0.
• Integration run of both follow-ups with Speed up assert bookkeeping: cached node paths and lazy stacks #24 (22eb4cd): 287 passed, 3 expected skips on each of Linux arm64/Python 3.10–3.14.
• Black, Flake8, MyPy, and Isort pass.

This removes unnecessary work on the rule-exception path. It does not change successful validation or claim to resolve all of #10.