sdbus++: catch elements with non-string names
sdbus++: catch elements with non-string names

The Python YAML parser turns certain keywords into non-string types,
such as 'On' -> True, 'Off' -> False.  If these aren't identified,
interfaces can be defined for which names become the
string-representation of these non-string names.  This is especially
problematic for 'On'/'Off' becoming 'True'/'False'.

Catch this condition and report an assertion.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: I126056e3eac9f97ebd7b095962da2b765046b532

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus