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/NAMESPACE b/NAMESPACE
index 9420cc0..bb89cd8 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -54,6 +54,7 @@
 importFrom(PTXQC,lcpCount)
 importFrom(PTXQC,lcsCount)
 importFrom(broom,tidy)
+importFrom(curl,has_internet)
 importFrom(dplyr,.data)
 importFrom(dplyr,anti_join)
 importFrom(dplyr,arrange)