ListAcInError: show error code for all energy meters, not just Fronius/CG

[cgoudie]

Summary

• The error code row in ListAcInError.qml is currently only visible for Fronius PV inverters and Carlo Gavazzi energy meters (hard-coded ProductId check). Third-party energy meter drivers (e.g. com.victronenergy.grid services) that publish /ErrorCode cannot surface errors in the device page.
• This change makes the error row visible for any device that publishes a non-zero /ErrorCode, while preserving existing Fronius and Carlo Gavazzi behaviour unchanged.
• Adds support for an optional /ErrorMessage D-Bus path. When a device publishes both /ErrorCode and /ErrorMessage, the GUI displays "<message> (<code>)" (e.g. "Open Neutral (5)"). Devices that only publish /ErrorCode get the raw numeric code displayed, matching the existing Fronius behaviour.

Motivation

We maintain a Venus OS driver (dbus-power-watchdog) for Hughes Power Watchdog energy meters. These devices report error codes (0-9) via BLE, which the driver publishes to /ErrorCode on its com.victronenergy.grid service. However, the error code is invisible on the Cerbo GX display because ListAcInError.qml only renders the row for two specific product IDs.

As a workaround we currently set our /ProductId to the Fronius value (0xA142) to make the error visible. This PR would let us (and other third-party drivers) use our own product ID while still having errors shown in the GUI.

Changes

components/listitems/ListAcInError.qml

1. Added a generic else branch after the Fronius and Carlo Gavazzi checks — shows the raw error code for any other product
2. Extended preferredVisible to include dataItem.valid && dataItem.value > 0 (non-zero error code present) and errorMessage.valid (device has a status message)
3. Added a VeQuickItem binding to /ErrorMessage — an optional D-Bus path that devices can publish to provide a human-readable error description

Backward compatibility

• Fronius: Unchanged — still shows raw error code, always visible
• Carlo Gavazzi: Unchanged — still shows translated messages, always visible
• All other devices: Error row appears only when /ErrorCode > 0 or /ErrorMessage is non-empty; hidden otherwise
• No changes to any other files

Test plan

• Verify Fronius PV inverter error display is unchanged
• Verify Carlo Gavazzi energy meter error display is unchanged
• Verify a third-party grid meter publishing /ErrorCode = 5 shows "5" in the error row
• Verify a third-party grid meter publishing /ErrorCode = 5 and /ErrorMessage = "Open Neutral" shows "Open Neutral (5)"
• Verify a third-party grid meter publishing /ErrorCode = 0 (no error) hides the error row
• Verify a device with no /ErrorCode path does not show the error row