[join] clarify ensureLoaded status msg

Was a confusing message to see while the joined sheet was loading.

visidata/features/join.py

@@ -11,7 +11,8 @@
 def ensureLoaded(vd, sheets):
     threads = [vs.ensureLoaded() for vs in sheets]
     threads = [t for t in threads if t]
-    vd.status('loading %d sheets' % len(threads))
+    if threads:
+        vd.status('finishing loading %d source sheets' % len(threads))
     return threads