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.

ci/test/06_script_b.sh

@@ -69,6 +69,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
           " src/util/bytevectorhash.cpp"\
           " src/util/check.cpp"\
           " src/util/error.cpp"\
+          " src/util/exception.cpp"\
           " src/util/getuniquepath.cpp"\
           " src/util/hasher.cpp"\
           " src/util/message.cpp"\