docs: add exception reference and error handling guide

[TheSaiEaranti]

Fixes #496

Adds docs/api/exceptions.rst and links it from the API toctree, so the exception hierarchy has one discoverable home.

What's on the page

• The hierarchy — all five classes in redisvl.exceptions, and a note that redis-py exceptions such as redis.exceptions.ConnectionError are not part of it. RedisVL wraps those and chains the original with raise ... from e, so __cause__ still holds it. That seemed like the detail most likely to trip someone writing an except clause.
• A table mapping each exception to the operations that raise it, e.g. SchemaValidationError → load(), RedisSearchError → create() / delete() / search() / aggregate().
• Four try/except examples, including the two the issue asks for.
• Autodoc reference for each class, following the :members: / :show-inheritance: pattern used in schema.rst.

Examples are drawn from real raise sites

Rather than writing plausible-looking snippets, I traced each exception to where it is actually raised so the examples match current behaviour:

Exception	Verified raise site	What the example shows
SchemaValidationError	storage.py _preprocess_and_validate_objects	validate_on_load=True, since validation is off by default and the error cannot otherwise occur
QueryValidationError	index.py _validate_query	ef_runtime on a flat vector field — the concrete case that method rejects
RedisSearchError	create, delete, search, aggregate	wrapping a Redis-side failure
RedisModuleVersionError	_check_svs_support, reached from create()	ordering except clauses, since it subclasses RedisVLError and not RedisSearchError

The from_yaml(..., validate_on_load=True) form matches the example already in the SearchIndex docstring.

Definition of Done

• Exception docs present and discoverable — docs/_build/html/api/exceptions.html renders, all five classes appear with their docstrings, RedisModuleVersionError.for_svs_vamana is documented, and the page is linked from the API index as "Exceptions".
• make docs-build passes — exit 0. The build reports 11 warnings; all 11 are pre-existing and point at query.py / aggregate.py docstrings inherited from redis-py, index.md heading levels, and the MCP how-to cross-references. None reference exceptions.rst.
• Examples align with current exception classes — per the table above.

codespell is clean on both files. No Python changed, so no behaviour change.

Notes

• Docs-only, so this needs the auto:documentation label per CONTRIBUTING. I can't set labels as an outside contributor — could a maintainer add it?
• I deliberately left uv.lock out. Running make docs-build with a recent uv rewrites it to lockfile revision 3 (a ~250 line diff) which has nothing to do with this change.
• If docs: migrate from Sphinx to mkdocs material #608 (Sphinx → mkdocs) lands first, this page is a single autodoc file and cheap to port; happy to do that conversion if you'd prefer to wait for it.

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Note

Low Risk
Documentation-only change with no runtime or API behavior impact.

Overview
Adds docs/api/exceptions.rst as the API home for RedisVL’s custom errors and wires it into the API toctree via docs/api/index.md.

The new page documents the RedisVLError hierarchy (including how redis-py errors are wrapped and chained on __cause__), a table mapping each exception to typical load() / query() / create() entry points, four try/except examples aligned with real raise sites, and autodoc blocks for all five classes in redisvl.exceptions.

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