Prepare fixes (#416)

* Error on overwrite=false but no output file

* Add prepare requirements to requirements.txt

* no saved output error -> warning

* Catch clustering import error

* lint fixes

* Fix missing output message

setup.py

@@ -26,5 +26,8 @@ setup(
     entry_points={
         "console_scripts":
             ["cellxgene = server.cli.cli:cli"]
-    }
+    },
+    extras_require=dict(
+        louvain=['python-igraph', 'louvain>=0.6'],
+    ),
 )