fix(Migrator): prevent nil pointer dereference in AlterColumn

[lihan3238]

Closes #338

Summary

AlterColumn has two issues:

1. Error from ColumnTypes() is discarded (assigned to _)
2. fieldColumnType is dereferenced without nil check when the schema field DBName is not found in the column types result

When schema/DB drift occurs (e.g. concurrent DROP COLUMN, partially applied migration), this causes a nil pointer panic.

Fix

• Propagate ColumnTypes() error instead of discarding it
• Add nil guard for fieldColumnType with a descriptive error message

Test Plan

• Follow the reproduction from Repro: nil pointer dereference in (Migrator).AlterColumn when schema field has no matching DB column playground#849
• Before: panic at fieldColumnType.DatabaseTypeName()
• After: returns descriptive error