Add FAILED: prefix to RBE Fatal error message

This error message it currently being
logged to stdout which goes to build.log file . We want it to be logged to
build_error.log file instead so that downstream systems that need to run
based on the error message will continue to work.

Test: Tested by running `m toybox` and ensuring that FAILED: prefix is
at the beginning of the line.

Bug: b/177221705
Change-Id: Ie0c9acee2739acf149384e33804b5fca3273c24f
diff --git a/ui/build/rbe.go b/ui/build/rbe.go
index d9c33f6..1fabd92 100644
--- a/ui/build/rbe.go
+++ b/ui/build/rbe.go
@@ -106,7 +106,7 @@
 	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)
 	}
 }