WebApp_PPS should not deliver 5v if the power supply is not PPS capable

[doniks]

previously discussed here: https://github.com/tooyipjee/Spark-Analyzer/issues/2

it seems that the WebApp_PPS falls back to delivering 5V if the power supply is not PPS capable. This could lead to people supplying higher voltage than what they want/anticipate.

I think the ideal solution would be to:

• detect that PPS is not supported
• show this in the web app
• turn off power output
• maybe the led can indicate some error state (does it support different colors? or rapid blinking)