Move AWSVPC/IPAM check to Go code

The weave script check was broken in an earlier commit.
The Go error message is less specific because it checks more cases.

prog/weaver/main.go

@@ -316,6 +316,9 @@ func main() {
 	if bridgeConfig.AWSVPC && len(config.Password) > 0 {
 		Log.Fatalf("--awsvpc mode is not compatible with the --password option")
 	}
+	if bridgeConfig.AWSVPC && !ipamConfig.Enabled() {
+		Log.Fatalf("--awsvpc mode requires IPAM enabled")
+	}
 
 	db, err := db.NewBoltDB(dbPrefix)
 	checkFatal(err)

weave

@@ -1252,10 +1252,6 @@ launch() {
             exit 1
         fi
     else
-        if [ -n "$AWSVPC" -a -z "$IPRANGE" ] ; then
-            echo "ERROR: Empty --ipalloc-range is not compatible with --awsvpc." >&2
-            exit 1
-        fi
         if [ -n "$IPRANGE" ] && ! check_overlap $IPRANGE ; then
             echo "WARNING: Specified --ipalloc-range $IPRANGE overlaps with existing route on host." >&2
             echo "Unless this is deliberate, you must pick another range and set it on all hosts." >&2