Fix multiple C++ bindings warnings with gcc 8.

Fix multiple C++ bindings warnings with gcc 8.

  bindings/cxx/classes.cpp: In function ‘int sigrok::call_log_callback(void*, int, const char*, __va_list_tag*)’:
  bindings/cxx/classes.cpp:242:17: warning: catching polymorphic type ‘class sigrok::Error’ by value [-Wcatch-value=]
    } catch (Error e) {
                   ^
  In file included from bindings/cxx/classes.cpp:1667:
  bindings/cxx/enums.cpp: In static member function ‘static Glib::VariantBase sigrok::ConfigKey::parse_string(std::__cxx11::string, sr_datatype)’:
  bindings/cxx/enums.cpp:789:13: warning: catching polymorphic type ‘class std::invalid_argument’ by value [-Wcatch-value=]
      } catch (invalid_argument) {
               ^~~~~~~~~~~~~~~~
  bindings/cxx/enums.cpp:804:13: warning: catching polymorphic type ‘class std::invalid_argument’ by value [-Wcatch-value=]
      } catch (invalid_argument) {
               ^~~~~~~~~~~~~~~~