dawn_node: Improve error handling for run-cts

Rebuild if the `out-node` directory is missing.
Actually check the server's status code.
Fix error handling for bad statuses.

Change-Id: I2164bba528036d39f10852f4ffd8ace76f145dba
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/67780
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/dawn_node/tools/src/cmd/run-cts/main.go

@@ -23,6 +23,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"math"
 	"net/http"
 	"os"
@@ -174,7 +175,8 @@ func run() error {
 		return fmt.Errorf("failed to scan source files for modified timestamps: %w", err)
 	}
 
-	ctsNeedsRebuild := mostRecentSourceChange.After(cache.BuildTimestamp)
+	ctsNeedsRebuild := mostRecentSourceChange.After(cache.BuildTimestamp) ||
+		!isDir(filepath.Join(r.cts, "out-node"))
 	if build {
 		if verbose {
 			fmt.Println("CTS needs rebuild:", ctsNeedsRebuild)
@@ -491,6 +493,7 @@ func (r *runner) runServer(caseIndices <-chan int, results chan<- result) error
 			continue
 		}
 
+		if postResp.StatusCode == http.StatusOK {
 			var resp Response
 			if err := json.NewDecoder(postResp.Body).Decode(&resp); err != nil {
 				res.error = fmt.Errorf("server response decode failure")
@@ -513,7 +516,16 @@ func (r *runner) runServer(caseIndices <-chan int, results chan<- result) error
 				res.status = skip
 				res.message = resp.Message
 			default:
-			err = fmt.Errorf("unknown status: '%v'", resp.Status)
+				res.status = fail
+				res.error = fmt.Errorf("unknown status: '%v'", resp.Status)
+			}
+		} else {
+			msg, err := ioutil.ReadAll(postResp.Body)
+			if err != nil {
+				msg = []byte(err.Error())
+			}
+			res.status = fail
+			res.error = fmt.Errorf("server error: %v", string(msg))
 		}
 		results <- res
 	}