No fallback if bus fails to be added in 0.16

[DedeHai]

What happened?

At bootup when reading the config and the number of LEDs is set such that they exceed MAX_LED_MEMORY no bus is added. This results in two problems:

• no LED output at all
• UI fails to load as there seems to be a required json element missing

To Reproduce Bug

I switched between different branches, that used the old and the new way of calculating LED memory on ESP8266 but I guess it will also work with:

• start with 0.15, add 1000 WS28x
• update to Fix LED buffer size calculation #4928

Expected Behavior

If a digital bus fails to be added at bootup, instead of exiting the loop its size should be truncated to the default amount of LEDs (30) instead of exiting the loop. This will overrun the MAX_LED_MEMORY slightly but that is the lesser evil. The users will need to adjust anyway.
Also the UI should not fail to load with an error (in index.js) if there is missing JSON elements, it should handle it more gracefully.

Install Method

Self-Compiled

What version of WLED?

0.16 PR

Which microcontroller/board are you seeing the problem on?

ESP8266

Relevant log/trace output

web UI: TypeError: cannot read properties of null

Anything else?

No response

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