go/pki: show helpful hint to new contributors about -hspki_disable

Change-Id: I714f503f5962578055b99009aa0eccb72517d37b
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1562
Reviewed-by: q3k <q3k@hackerspace.pl>
diff --git a/go/pki/locate.go b/go/pki/locate.go
index e075f55..1d7e45c 100644
--- a/go/pki/locate.go
+++ b/go/pki/locate.go
@@ -104,5 +104,5 @@
 	}
 	glog.Warningf("Could not load flag-defined PKI credentials: %v", err)
 
-	return nil, fmt.Errorf("could not load any credentials")
+	return nil, fmt.Errorf("could not load PKI credentials (hint: run `prodaccess` to set up developer certs or add `-- -hspki_disable` to bazel run command)")
 }