fix(github): try/catch getPrList

lib/platform/github/index.ts

@@ -974,11 +974,17 @@ export async function getPrList(): Promise<Pr[]> {
   logger.trace('getPrList()');
   if (!config.prList) {
     logger.debug('Retrieving PR list');
-    const res = await api.get(
+    let res;
+    try {
+      res = await api.get(
         `repos/${config.parentRepo ||
           config.repository}/pulls?per_page=100&state=all`,
         { paginate: true }
       );
+    } catch (err) /* istanbul ignore next */ {
+      logger.info({ err }, 'getPrList err');
+      throw new Error('platform-failure');
+    }
     config.prList = res.body.map(
       (pr: {
         number: number;