feat: validate change history table schema upfront, add ERROR_MESSAGE column

[luisggc]

Summary

• Adds ERROR_MESSAGE VARCHAR to the CREATE TABLE statement so newly created change history tables include the column from the start
• Adds validate_change_history_schema() called at initialization (after the table is confirmed to exist, before any scripts run), using a fast metadata-only SELECT ERROR_MESSAGE FROM <table> LIMIT 0 check
• If the column is missing on an existing table (created by an older version of schemachange or manually by a DBA): schemachange attempts ALTER TABLE ... ADD COLUMN IF NOT EXISTS ERROR_MESSAGE VARCHAR automatically
• If the role lacks ALTER privileges, the deploy fails immediately with a clear message showing the exact SQL to run manually — no mid-deploy surprises
• In dry-run mode, a warning is logged if the column is missing but no ALTER is attempted
• Removes the previous runtime try/except hack in record_change_history that added the column on INSERT failure

Motivation

Previously, ERROR_MESSAGE was added via auto-ALTER when an INSERT failed at runtime. This was risky: it introduced DDL mid-deploy, and if the role lacked ALTER privileges the deploy would break even for successful scripts (since error_message='' is always passed). Moving this check to initialization gives a predictable, upfront failure instead.

This is a prerequisite for the continue-on-error feature (#339).