sbbecho: Add more descriptive error messages

2025-02-19 00:25:14 import_netmail(/sbbs/netmail/1.msg) returned 4

Unfortunately, "4" doesn't mean much.

assigned to @rswindell

It's not an error, it's just a debug-level log message. In that context, it means (netmail) "Created locally". More details would be output to stdout, but we don't usually log the stdout of sbbsecho, so yes, we could log more in that context or just remove the debug-level message.