Handle errors in populatingAdapter function(s)

Summary

We are currently ignoring error values in some methods attached to the populatingAdapter that translates Helm chart values to values the operator can use, https://gitlab.com/gitlab-org/cloud-native/gitlab-operator/-/blob/master/controllers/gitlab/adapter.go#L287. We should handle errors so we can bubble them up to functions that call populateValues - otherwise if something goes wrong it can fail silently.

This was discussed in !450 (comment 917227548).