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.
The `GUIUtil::ObjectInvoke()` template function was a replacement of
the `QMetaObject::invokeMethod()` functor overload which is available
in Qt 5.10+.
No behavior change.
