Emit Unknown event on tags not known by PLC.

[GeekMeUpScotty]

Description, Motivation, and Context

Emit Unknown event when PLC is queried for an unknown tag.

Currently the PLC would go offline and reconnect whenever an unknown tag was attempted to be read.
This PR will ensure that PLC does not go offline and will emit an event (Unknown). Which is catchable in "node-red" (eg. node-red-contrib-cip-st-ethernet-ip)

How Has This Been Tested?

Intentionally set up Node-Red to query non-existing tags.
Modified to node-red-contrib-cip-st-ethernet-ip to catch the event. Verified that PLC is no-longer going in and out of offline.

Screenshots (if appropriate):

Types of changes

• [ x] Bug fix (non-breaking change which fixes an issue) (No reason to go offline on non-existing tag[s])
• [ x] New feature (non-breaking change which adds functionality) (Emit event on unknown)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• [?] All new and existing tests passed.
• This is a work in progress, and I want some feedback (If yes, please mark it in the title -> e.g. [WIP] Some awesome PR title)

Related Issue

#44