Display "No VTX Input" in Goggles when there is a MIPI/Camera problem

[MartinHugh]

Problem
When a vtx cannot detect input (usually bad camera/MIPI) it is hard to distinguish from other faults or problems which might prevent the goggles receiving an image from the vtx. A precedent for this particular class of fault already exists in projector and stadium display systems.

Suggested Solution
If a vtx cannot detect input, it should transmit "No input to VTX" or similar to the goggles.

Suggested Implementation
When this fault is present the vtx should send the colourbar diagnostic raster, and draw "No input to vtx" on the OSD matrix.

The user will then be clear about where the problem lies, i.e. not in the vtx output or its configuration, and not in the goggles, but (usually) in the MIPI cable or camera. This should make diagnosing and fixing this very common problem much easier

[pitts-mo]

This seems like a good solution. Possibly a better / more ideal solution than existing issue #142 suggests.