Fail action execution on unhandled errors

Without this, the error logs contain an "UnhandledPromiseRejectionError"
but the action is reported as succeeding.

src/post.ts

@@ -1,8 +1,16 @@
+import * as core from '@actions/core'
 import * as caches from './caches'
 
 // Invoked by GitHub Actions
 export async function run(): Promise<void> {
-    await caches.save()
+    try {
+        await caches.save()
+    } catch (error) {
+        core.setFailed(String(error))
+        if (error instanceof Error && error.stack) {
+            core.info(error.stack)
+        }
+    }
 }
 
 run()