Update mockttp to report upstream TLS errors

Author: pimterry

package-lock.json

@@ -43,7 +43,7 @@
     "graphql": "^14.0.2",
     "graphql-yoga": "^1.18.1",
     "lodash": "^4.17.13",
-    "mockttp": "^0.18.1",
+    "mockttp": "^0.19.0",
     "node-gsettings-wrapper": "^0.5.0",
     "registry-js": "^1.4.0",
     "rimraf": "^2.6.2",