Fix dns_netcup reporting a bogus 4013 instead of the real zone error

The zone lookup walked the challenge name from the right and ended up
asking netcup for the full "_acme-challenge.<domain>" as a zone name.
That can never be a zone, so netcup answered 4013 "Validation Error",
which replaced the real 5028 "The zone <domain> could not be found" as
the error shown to the user.

Stop one label short of the full name, and fail explicitly when no zone
matched, reporting the last API response plus what to check. Before, a
run where every candidate returned 5028 fell through to logout and
returned success.

dnsapi/dns_netcup.sh

@@ -33,9 +33,11 @@ dns_netcup_add() {
   exit=$(echo "$fulldomain" | tr -dc '.' | wc -c)
   exit=$(_math "$exit" + 1)
   i=$exit
+  _nc_last=$(_nc_lastlevel "$i")
+  _nc_found=""
 
   while
-    [ "$exit" -gt 0 ]
+    [ "$exit" -ge "$_nc_last" ]
   do
     tmp=$(echo "$fulldomain" | cut -d'.' -f"$exit")
     if [ "$(_math "$i" - "$exit")" -eq 0 ]; then
@@ -51,12 +53,18 @@ dns_netcup_add() {
           _err "$msg"
           return 1
         else
+          _nc_found=1
           break
         fi
       fi
     fi
     exit=$(_math "$exit" - 1)
   done
+  if [ -z "$_nc_found" ]; then
+    _err "$msg"
+    _nc_nozone "$fulldomain"
+    return 1
+  fi
   logout
 }
 
@@ -70,9 +78,11 @@ dns_netcup_rm() {
   exit=$(_math "$exit" + 1)
   i=$exit
   rec=""
+  _nc_last=$(_nc_lastlevel "$i")
+  _nc_found=""
 
   while
-    [ "$exit" -gt 0 ]
+    [ "$exit" -ge "$_nc_last" ]
   do
     tmp=$(echo "$fulldomain" | cut -d'.' -f"$exit")
     if [ "$(_math "$i" - "$exit")" -eq 0 ]; then
@@ -89,12 +99,18 @@ dns_netcup_rm() {
           _err "$msg"
           return 1
         else
+          _nc_found=1
           break
         fi
       fi
     fi
     exit=$(_math "$exit" - 1)
   done
+  if [ -z "$_nc_found" ]; then
+    _err "$msg"
+    _nc_nozone "$fulldomain"
+    return 1
+  fi
 
   ida=0000
   idv=0001
@@ -125,6 +141,27 @@ dns_netcup_rm() {
   logout
 }
 
+# The zone is looked up by walking the challenge name from the right, one
+# label at a time. The leftmost label is the challenge prefix, so the full
+# name itself can never be a zone: asking netcup for it only returns 4013
+# "Validation Error", which would then mask the real 5028 "zone could not be
+# found". Stop one label short, unless the name is too short to have a
+# challenge prefix at all (manual invocation).
+# levels
+_nc_lastlevel() {
+  if [ "$1" -ge 3 ]; then
+    echo 2
+  else
+    echo 1
+  fi
+}
+
+# fulldomain
+_nc_nozone() {
+  _err "No DNS zone for $1 was found at netcup."
+  _err "Check that the domain belongs to the account of the configured NC_CID and that its DNS is hosted at netcup."
+}
+
 _login() {
   tmp=$(_post "{\"action\": \"login\", \"param\": {\"apikey\": \"$NC_Apikey\", \"apipassword\": \"$NC_Apipw\", \"customernumber\": \"$NC_CID\"}}" "$end" "" "POST")
   sid=$(echo "$tmp" | tr '{}' '\n' | grep apisessionid | cut -d '"' -f 4)