config_parse.py: add _append_error_ctx() to remove redundant code

config_parse.py: add _append_error_ctx() to remove redundant code

_append_error_ctx() appends context to a given `ConfigParseError`
exception and then reraises it.

Using

    catch ConfigParseError as exc:
        _append_error_ctx(exc, 'My object', 'My message')

instead of

    catch ConfigParseError as exc:
        exc.append_ctx('My object', 'My message')
        raise

makes sure we don't forget to reraise.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>