warn on -iprange overlap

Closes #1276.

weave

@@ -1041,10 +1041,15 @@ case "$COMMAND" in
         if [ -z "$IPRANGE_SPECIFIED" ] ; then
             IPRANGE="10.128.0.0/10"
             if command_exists netcheck && ! netcheck $IPRANGE ; then
-                echo "Default -iprange $IPRANGE overlaps with existing route." >&2
-                echo "Please pick another range and set it on all hosts." >&2
+                echo "ERROR: Default -iprange $IPRANGE overlaps with existing route." >&2
+                echo "You must pick another range and set it on all hosts." >&2
                 exit 1
             fi
+        else
+            if command_exists netcheck && ! netcheck $IPRANGE ; then
+                echo "WARNING: Specified -iprange $IPRANGE overlaps with existing route." >&2
+                echo "Unless this is deliberate, you must pick another range and set it on all hosts." >&2
+            fi
         fi
         # Set WEAVE_DOCKER_ARGS in the environment in order to supply
         # additional parameters, such as resource limits, to docker