refactor: avoid unreachable throw in `inline_check_non_fatal` #34426

In builds where G_ABORT_ON_FAILED_ASSUME is true, inline_check_non_fatal called assertion_fail() and then had a throw NonFatalCheckError that was never reachable, because assertion_fail() either aborts or throws and never returns. Refactored inline_check_non_fatal to use if constexpr (G_ABORT_ON_FAILED_ASSUME) { assertion_fail(...) } else { throw NonFatalCheckError(...) } branch, removing the unreachable throw

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34426.

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Thank you for your contribution. While this stylistic change can make sense on its own, it comes at a cost and risk for the project as a whole. The weak motivation for the change does not justify the burden that it places on the project. A burden could be any of the following:

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• Accidental introduction of bugs
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