fix: replace assert with UpdateError in Table.update() method

[KooshaPari]

Bug

Table.update() used a bare assert rowcount == 1 statement to validate the update affected exactly one row. This has two problems:

1. assert statements are stripped when Python runs in optimized mode (python -O), silently disabling the check
2. The error message when the assert fails (AssertionError) is not informative

The code had a TODO comment acknowledging this: # TODO: Test this works (rolls back) - use better exception

Fix

• Replaced assert rowcount == 1 with an explicit check that raises a new UpdateError exception with a descriptive message
• Added UpdateError exception class to sqlite_utils/db.py

Testing

• Added test_update_error_class_exists to verify the exception class works
• All 20 tests in test_update.py pass

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📚 Documentation preview 📚: https://sqlite-utils--867.org.readthedocs.build/en/867/

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