go.tools/dashboard/builder: do not fatal on subrepo fetch errors

When fetching repos it is not uncommon for a 500 or 503 to be returned
from code.google.com. When this happens, log the error and continue so
that we try again later, rather than treating this as fatal.

R=adg, dvyukov
CC=golang-dev
https://golang.org/cl/38720044

dashboard/builder/main.go

@@ -492,7 +492,8 @@ func commitWatcher(goroot *Repo) {
 		for _, pkg := range dashboardPackages("subrepo") {
 			pkgmaster, err := vcs.RepoRootForImportPath(pkg, *verbose)
 			if err != nil {
-				log.Fatalf("Error finding subrepo (%s): %s", pkg, err)
+				log.Printf("Error finding subrepo (%s): %s", pkg, err)
+				continue
 			}
 			pkgroot := &Repo{
 				Path:   filepath.Join(*buildroot, pkg),