use EXIT_ codes instead of magic numbers for exit(...) and main return values

[theStack]

This is really only a minor issue, but I noticed while reviewing #1479 that the return codes of functions in the examples could potentially be confusing. Throughout the API and internal functions we use 0=failure/1=success, while for the main function and (exit(...)) it's the other way round, i.e. 0=success/1=failure. We could use EXIT_{SUCCESS,FAILURE} (defined in stdlib.h, see https://en.cppreference.com/w/c/program/EXIT_status) for the latter instead for more clarity.

See e.g. bitcoin/bitcoin@4441018 for a comparable change in Bitcoin Core as orientation. This could be a good first issue.

[real-or-random]

Concept ACK

I had the same thought in the past.

My suggestion to remove assert() in the examples is slightly related and could be addressed in another commit in the same PR that would resolve this issue here.