Return "not found" on any `getblockhash` exit status. Previously, only exit code 8 (block height doesn't exist) returned "not found", while other exit codes returned an error. Now any non-zero exit status returns "not found" since any failure means the block is unavailable.
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).
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