* lang/cpp/src/error.h (Error::asString): Mark as deprecated.
* lang/cpp/src/context.cpp (operator<<),
lang/cpp/src/editinteractor.cpp (edit_interactor_callback_impl),
lang/cpp/tests/run-getkey.cpp (main),
lang/cpp/tests/run-keylist.cpp (main),
lang/cpp/tests/run-wkdlookup.cpp (main): Use Error::asStdString instead
of Error::asString.