Improve error message when an invalid enum value is provided. #605
Description
When an invalid enum value is provided in a config, the string_to_enum method will simply say:
stoi
It would be nice if we could improve this error message to be a bit more informative. For example, along the lines of:
Invalid enum value: 'bart'. Must be one of [red, green, blue, yellow].
I have already implemented this change; I'll link the PR shortly. Let me know if this looks good.