Add Custom Exception Hierarchy and Fix Silent Swallowing — pyrite

Two related problems:

1. No custom exceptions — the codebase uses generic `ValueError` everywhere. A hierarchy like `PyriteError > EntryNotFoundError, ValidationError, PluginError, StorageError` would make error handling precise and let callers catch specific failures.

2. Broad `except Exception` with silent `pass` — ~74 instances across the codebase. The worst offenders:

`schema.py` validation: three nested try/except with bare `pass` — makes plugin validator debugging impossible

`database.py` `_create_plugin_tables()`: swallows table creation failures silently, causing cryptic SQL errors later

`core_types.py` `get_entry_class()`: silently falls back to GenericEntry if registry is broken

`services/kb_service.py` `_run_hooks()`: returns original entry on exception instead of propagating, defeating before-hooks that should abort operations

Fix: replace bare `pass` with `logger.warning()` at minimum; use custom exceptions where callers need to distinguish error types.

Add Custom Exception Hierarchy and Fix Silent Swallowingbacklog_item

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