Add messages when variables are not available listings

[fukuhiro2023]

This PR updates prepare_ae_forestly() to:
1. raise an error when the population and observation terms are the same and the variables specified in var for population do not exist in data_observation; and
2. issue a warning when display_ae_listing variables do not exist in data_observation, and display only the variables that do exist in data_observation.

[wangben718]

Looks good to me!

[fukuhiro2023]

Based on our discussion, I updated prepare_ae_forestly() to merge data_observation and data_population. As a result, the case where the population and observation terms are the same, but variables specified in var for the population do not exist in data_observation, no longer needs to be considered. Therefore, Update 1 above has been replaced. Update 2 remains to handle cases where non-existent variables may be specified in display_ae_listing.