Groups resource listing uses invalid URLs after error

_Noticed odd behavior when reviewing https://github.com/nextstrain/nextstrain.org/pull/1274, but this behavior is reproducible on prod._ After landing on a non-existent Groups dataset, the Groups resource listing uses invalid URLs. 1. Go to https://nextstrain.org/groups/blab/foo 2. See error message for non-existent dataset 3. Scroll down to click on a dataset, e.g. 229/E, and get directed to bad URL https://nextstrain.org/groups/blab/blab/229e/E