ch04 preprocessing: add Check for not supported value and error handlers

[WMP]

Summary

Adds a new subsection "Check for not supported value and error handlers" to
the preprocessing page (ch04-zabbix-collecting-data/preprocessing.md).

The chapter already lists Check for not supported value among the available
preprocessing steps, but never explains it, and error handlers (Custom on fail)
are not documented anywhere in the book. This fills that gap.

What it adds

• How an item failing to collect a value (SNMP error, unreadable file,
  unsupported agent key) becomes not supported, and how that — together with
  triggers going to unknown — generates internal events that bloat the
  events/event_tag tables and overload the housekeeper.
• The Check for not supported value step (runs because the item failed).
• Error handlers / Custom on fail: Discard value, Set value to,
  Set error to.
• The practical recipe: Check for not supported value → Discard value to stop
  a not-supported flood at the source.
• Three real-world examples: virtual network interfaces returning invalid
  argument on /sys/class/net/<if>/speed, empty SNMP hardware slots returning
  genError, and a Prometheus pattern step failing when the source object
  (e.g. a pod) disappears (handled via the step's own error handler, since the
  collection itself succeeded).
• A note that LLD filters are the cleaner first line of defence, and a
  caution that discarding errors hides them, so it should be used deliberately.

Why

These steps are the standard, supported way to keep a large environment's
database from filling with internal-event noise, yet they are easy to miss.
Documenting them with concrete examples makes the behaviour discoverable.

Notes

• Style follows the book conventions (prose, ### subsection, !!! tip
  admonition, ~80-column wrapping). One file changed; no images required.

Sources

• Zabbix manual — Item value preprocessing (incl. custom error handling):
  https://www.zabbix.com/documentation/current/en/manual/config/items/preprocessing
• Zabbix manual — Items / item states (not supported):
  https://www.zabbix.com/documentation/current/en/manual/config/items/item
• Zabbix manual — Event sources (internal events):
  https://www.zabbix.com/documentation/current/en/manual/config/events/sources
• Zabbix manual — Low-level discovery (LLD filters):
  https://www.zabbix.com/documentation/current/en/manual/discovery/low_level_discovery
• Behaviour verified against a large production environment (k8s/SNMP templates
  generating millions of internal events/day from virtual or absent entities).