Add FAILED: prefix to RBE Fatal error message"

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1560871

Bug: b/195592452
Change-Id: I00e2cb26eb3e1e3b695ba067cad24f45169f3a58
(cherry picked from commit ab2803a870)
Merged-In: Ie0c9acee2739acf149384e33804b5fca3273c24f

ui/build/rbe.go

@@ -81,7 +81,7 @@ func startRBE(ctx Context, config Config) {
 	cmd := Command(ctx, config, "startRBE bootstrap", rbeCommand(ctx, config, bootstrapCmd))
 
 	if output, err := cmd.CombinedOutput(); err != nil {
-		ctx.Fatalf("rbe bootstrap failed with: %v\n%s\n", err, output)
+		ctx.Fatalf("Unable to start RBE reproxy\nFAILED: RBE bootstrap failed with: %v\n%s\n", err, output)
 	}
 }