Make sure to always fail gracefully if server not accessible

See CRAN policy:

Packages which use Internet resources should fail gracefully with an informative message
if the resource is not available or has changed (and not give a check warning nor error).

at https://cran.r-project.org/web/packages/policies.html

Change-Id: I0c346c75786b8f5392694337254d6f19f91d7caf
diff --git a/R/ci.R b/R/ci.R
index a0c04f7..19f8615 100644
--- a/R/ci.R
+++ b/R/ci.R
@@ -40,8 +40,12 @@
   x <- enquo(x)
   N <- enquo(N)
   nas <- df %>%
-    dplyr::filter(total <= 0) %>%
+    dplyr::filter(is.na(total) | total <= 0) %>%
     mutate(f = NA, conf.low = NA, conf.high = NA)
+
+  if (nrow(df) == nrow(nas))
+    return(nas)
+
   df %>%
     dplyr::filter(total > 0) %>%
     rowwise %>%