Echo error to stderr

bin/vultr/vcurl

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 if [[ -z "${VULTR_API_KEY}" ]] ; then
-  echo "VULTR_API_KEY not defined in environment"
+  >&2 echo "VULTR_API_KEY not defined in environment"
   exit 1
 fi