Make Zabbix agent argument errors say which fix is needed - #4

The report

Running the Linux Zabbix agent script from TacticalRMM against the newly onboarded Komune / Hotel site failed with:

ERROR: ZabbixProxy and ZabbixServer are required.

Accurate, but it doesn't say what to do — and it fires for three different causes that need three different fixes.

What actually happens (traced through TRMM's source, not guessed)

• parse_script_args (scripts/models.py:205) passes an argument through unaltered when the resolved value is falsy — so an undefined variable arrives as the literal {{site.ZabbixProxy}}.
• get_db_value (tacticalrmm/utils.py:320) returns the custom field's default_value when the site has no saved value — normally empty, which TRMM then substitutes as ''.

So the two cases are distinguishable at the script, and mean opposite things:

The observed error was the empty case — the ZabbixProxy / ZabbixServer Site custom fields exist but have no value for the new site.

Change

Each case now reports separately with the exact UI path, including the distinction that {{global.X}} resolves from the Key Store, not Custom Fields — an easy mix-up.

ERROR: ZabbixProxy is empty.
ERROR: ZabbixServer is empty.
       The Site custom field exists but has no value for this agent's site,
       so TacticalRMM substituted an empty string.
       Set it in: Clients -> <client> -> Sites -> <site> -> Edit -> Custom Fields
       Both ZabbixProxy and ZabbixServer must be set on every site you deploy to.

ERROR: TacticalRMM did not substitute ZabbixVersion — it arrived as: {{global.ZabbixVersion}}
       That means no 'ZabbixVersion' entry exists in the Key Store.
       Create it in: Settings -> Global Settings -> Key Store

All four paths (no args, unresolved site field, blank field, unresolved global) were exercised directly; the blank-field case reproduces the reported failure exactly. bash -n and shellcheck clean. No behaviour change for a correctly configured run.

Related trap found while reading that code — not fixed here

If a Site custom field is undefined, get_db_value falls through to attribute lookup and ends up returning the Site object, which stringifies to the site name. So {{site.ZabbixProxy}} would arrive as Hotel — which passes the script's address regex and would silently configure Server=Hotel.

The literal-{{...}} guard added here does not catch that, because TRMM does substitute a value. Options if you want it covered: warn when the address has no dot and is not an IP (risks false positives on short internal hostnames), or resolve it before writing the config. Happy to add either — flagging rather than guessing at the right trade-off.