Update to offer more helpful error message if str fallback is used wi…

[jwlodek]

…th enum

Addresses #750

[DominicOram]

Awesome, thank you for addressing this quickly! I've added a couple of nit-picks but they're just over style, take them or leave them

[DominicOram]

Could: I don't think the else statements are needed:

Suggested change

	if valid_choices is not None and len(valid_choices) > 0:
	if value not in valid_choices:
	msg = (
	f"{value} is not a valid choice for {self.write_pv}, "
	f"valid choices: {self.converter.metadata.get('choices')}"
	)
	raise ValueError(msg) from exc
	else:
	raise
	else:
	raise
	if valid_choices is not None and len(valid_choices) > 0:
	if value not in valid_choices:
	msg = (
	f"{value} is not a valid choice for {self.write_pv}, "
	f"valid choices: {self.converter.metadata.get('choices')}"
	)
	raise ValueError(msg) from exc
	raise