Add begin() guard

[sebromero]

This PR adds a guard to make begin() idempotent. Along with that the PR fixes a few warnings and adds additional error handling.

[github-actions]

Memory usage change @ e95199c

Board	flash	%	RAM for global variables	%
arduino:mbed_nicla:nicla_vision	🔺 +112 - +112	+0.01 - +0.01	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A
arduino:mbed_portenta:envie_m7:target_core=cm4	N/A	N/A	N/A	N/A
arduino:renesas_portenta:portenta_c33	🔺 +216 - +216	+0.01 - +0.01	🔺 +4 - +4	0.0 - 0.0

Click for full report table

Board	examples/BasicUsage flash	%	examples/BasicUsage RAM for global variables	%	examples/ReadWriteRegs flash	%	examples/ReadWriteRegs RAM for global variables	%
arduino:mbed_nicla:nicla_vision	112	0.01	0	0.0	112	0.01	0	0.0
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
arduino:mbed_portenta:envie_m7:target_core=cm4	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
arduino:renesas_portenta:portenta_c33	216	0.01	4	0.0	216	0.01	4	0.0

Click for full report CSV

Board,examples/BasicUsage<br>flash,%,examples/BasicUsage<br>RAM for global variables,%,examples/ReadWriteRegs<br>flash,%,examples/ReadWriteRegs<br>RAM for global variables,%
arduino:mbed_nicla:nicla_vision,112,0.01,0,0.0,112,0.01,0,0.0
arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A
arduino:mbed_portenta:envie_m7:target_core=cm4,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A
arduino:renesas_portenta:portenta_c33,216,0.01,4,0.0,216,0.01,4,0.0

[aentinger]

LGTM 👍