refactor: Extract util/exception from util/system

This is a minimal extraction of a single function, but also the only use
of std::exception in util/system.

The background of this commit is an ongoing effort to decouple the
libbitcoinkernel library from the ArgsManager defined in system.h.
Moving the function out of system.h allows including it from a separate
source file without including the ArgsManager definitions from system.h.

src/util/thread.cpp

@@ -5,7 +5,7 @@
 #include <util/thread.h>
 
 #include <logging.h>
-#include <util/system.h>
+#include <util/exception.h>
 #include <util/threadnames.h>
 
 #include <exception>