Vtgateproxy firstready balancer fail fast (#362)

* return a custom error to fail fast

* fatal if the resolver cannot initialize

go/vt/vtgateproxy/firstready_balancer.go

@@ -31,6 +31,7 @@ limitations under the License.
 // https://github.com/grpc/grpc-go/blob/master/pickfirst.go
 
 import (
+	"errors"
 	"sync"
 
 	"google.golang.org/grpc/balancer"
@@ -60,7 +61,7 @@ func (f *frPickerBuilder) Build(info base.PickerBuildInfo) balancer.Picker {
 	log.V(100).Infof("first_ready: Build called with info: %v", info)
 
 	if len(info.ReadySCs) == 0 {
-		return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
+		return base.NewErrPicker(errors.New("no available connections"))
 	}
 
 	f.mu.Lock()

go/vt/vtgateproxy/vtgateproxy.go

@@ -218,12 +218,16 @@ func Init() {
 		return append(opts, grpc.WithDefaultServiceConfig(fmt.Sprintf(`{"loadBalancingConfig": [{"%s":{}}]}`, *balancerType))), nil
 	})
 
-	RegisterJSONGateResolver(
+	_, err := RegisterJSONGateResolver(
 		*vtgateHostsFile,
 		*addressField,
 		*portField,
 		*poolTypeField,
 		*affinityField,
 		*affinityValue,
 	)
+
+	if err != nil {
+		log.Fatalf("error initializing resolver: %v", err)
+	}
 }