rpc: add unary+streaming interceptors to log all errors

Not all errors that occur when serving client requests in the gRPC
server are logged. As a result, at times, we can be lacking critic
information that can be used to debug issues that pop up. With this PR,
we create a basic streaming+unary interceptor that will log all errors
that occur when servicing calls.

The current format looks something like this in the logs when an error
occurs:

```
[ERR] RPCS: [/lnrpc.Lightning/SendCoins]: decoded address is of unknown format
```

log.go

@@ -1,6 +1,7 @@
 package lnd
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"os"
@@ -37,6 +38,7 @@ import (
 	"github.com/lightningnetwork/lnd/sweep"
 	"github.com/lightningnetwork/lnd/watchtower"
 	"github.com/lightningnetwork/lnd/watchtower/wtclient"
+	"google.golang.org/grpc"
 )
 
 // Loggers per subsystem.  A single backend logger is created and all subsystem
@@ -235,3 +237,36 @@ func (c logClosure) String() string {
 func newLogClosure(c func() string) logClosure {
 	return logClosure(c)
 }
+
+// errorLogUnaryServerInterceptor is a simple UnaryServerInterceptor that will
+// automatically log any errors that occur when serving a client's unary
+// request.
+func errorLogUnaryServerInterceptor(logger btclog.Logger) grpc.UnaryServerInterceptor {
+	return func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo,
+		handler grpc.UnaryHandler) (interface{}, error) {
+
+		resp, err := handler(ctx, req)
+		if err != nil {
+			// TODO(roasbeef): also log request details?
+			logger.Errorf("[%v]: %v", info.FullMethod, err)
+		}
+
+		return resp, err
+	}
+}
+
+// errorLogStreamServerInterceptor is a simple StreamServerInterceptor that
+// will log any errors that occur while processing a client or server streaming
+// RPC.
+func errorLogStreamServerInterceptor(logger btclog.Logger) grpc.StreamServerInterceptor {
+	return func(srv interface{}, ss grpc.ServerStream,
+		info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
+
+		err := handler(srv, ss)
+		if err != nil {
+			logger.Errorf("[%v]: %v", info.FullMethod, err)
+		}
+
+		return err
+	}
+}

rpcserver.go

@@ -560,6 +560,15 @@ func newRPCServer(s *server, macService *macaroons.Service,
 	unaryInterceptors := append(macUnaryInterceptors, promUnaryInterceptors...)
 	strmInterceptors := append(macStrmInterceptors, promStrmInterceptors...)
 
+	// We'll also add our logging interceptors as well, so we can
+	// automatically log all errors that happen during RPC calls.
+	unaryInterceptors = append(
+		unaryInterceptors, errorLogUnaryServerInterceptor(rpcsLog),
+	)
+	strmInterceptors = append(
+		strmInterceptors, errorLogStreamServerInterceptor(rpcsLog),
+	)
+
 	// If any interceptors have been set up, add them to the server options.
 	if len(unaryInterceptors) != 0 && len(strmInterceptors) != 0 {
 		chainedUnary := grpc_middleware.WithUnaryServerChain(