Return result from ArduinoLEDMatrix::begin() compatible with ArduinoGraphics.

[PhilLogan]

Current implementation returns no value, resulting in annoying compiler warning - change returns value based on ArduinoGraphics docs (https://www.arduino.cc/reference/en/libraries/arduinographics/begin/).

[CLAassistant]

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[aentinger]

And I'm seeing that in order to merge this, I need you to sign the Contributor License Agreement. Could you please do that?

[pennam]

@PhilLogan could you please sign the CLA?