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/qt/bitcoin.cpp

@@ -29,6 +29,7 @@
 #include <qt/utilitydialog.h>
 #include <qt/winshutdownmonitor.h>
 #include <uint256.h>
+#include <util/exception.h>
 #include <util/string.h>
 #include <util/system.h>
 #include <util/threadnames.h>

src/qt/guiutil.cpp

@@ -20,6 +20,7 @@
 #include <protocol.h>
 #include <script/script.h>
 #include <script/standard.h>
+#include <util/exception.h>
 #include <util/system.h>
 #include <util/time.h>
 

src/qt/initexecutor.cpp

@@ -5,7 +5,7 @@
 #include <qt/initexecutor.h>
 
 #include <interfaces/node.h>
-#include <util/system.h>
+#include <util/exception.h>
 #include <util/threadnames.h>
 
 #include <exception>