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/DESCRIPTION b/DESCRIPTION
index 257b420..a0e335c 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -45,6 +45,7 @@
     keyring,
     utils,
     httr,
+    curl,
     methods,
     PTXQC,
     purrr,