isolate: give up and die on file unavailability

client/isolate/isolate.go

Index: client/isolate/isolate.go
diff --git a/client/isolate/isolate.go b/client/isolate/isolate.go
index a0ddfed085defa634cc2067e612f0f6e7c377f98..bfb9f628f26a05cde7881fc2ca02c85891e09ad1 100644
--- a/client/isolate/isolate.go
+++ b/client/isolate/isolate.go
@@ -279,7 +279,9 @@ func archive(arch *archiver.Archiver, opts *ArchiveOptions, displayName string)
 			if mode&os.ModeSymlink == os.ModeSymlink {
 				l, err := os.Readlink(dep)
 				if err != nil {
-					return nil, err
+					// Kill the process: there's no reason to continue if a file is
+					// unavailable.
+					log.Fatalf("Unable to stat %q: %v", dep, err)
 				}
 				i.Files[relPath] = isolated.SymLink(l)
 			} else {