fix npm#249

This attaches a catch handler to the handleContentP promise, as in some cases the flush is not called before node decides that this is an unhandled promise rejection.

lib/content/write.js

@@ -67,6 +67,7 @@ class CacacheWriteStream extends Flush {
         this.cache,
         this.opts
       )
+      this.handleContentP.catch(error => this.destroy(error))
     }
     return this.inputStream.write(chunk, encoding, cb)
   }