How to get the Listener When thows ProxyCacheException

My network connection has suddenly broken down，it throw Exception,but I need to do something,How to get the Listener When it thows ProxyCacheException?
```
05-24 10:39:35.267 14916-15787/? E/HttpProxyCacheServer: HttpProxyCacheServer error
com.danikula.videocache.ProxyCacheException: Error processing request. Version: 2.7.0
at com.danikula.videocache.HttpProxyCacheServer.processSocket(HttpProxyCacheServer.java:241)
at com.danikula.videocache.HttpProxyCacheServer.access$200(HttpProxyCacheServer.java:52)
at com.danikula.videocache.HttpProxyCacheServer$SocketProcessorRunnable.run(HttpProxyCacheServer.java:338)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:423)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
at java.lang.Thread.run(Thread.java:818)
Caused by: com.danikula.videocache.ProxyCacheException: Error opening connection for xxx.mp4 with offset 7231527. Version: 2.7.0
at com.danikula.videocache.HttpUrlSource.open(HttpUrlSource.java:75)
at com.danikula.videocache.HttpProxyCache.responseWithoutCache(HttpProxyCache.java:89)
at com.danikula.videocache.HttpProxyCache.processRequest(HttpProxyCache.java:47)
at com.danikula.videocache.HttpProxyCacheServerClients.processRequest(HttpProxyCacheServerClients.java:42)
at com.danikula.videocache.HttpProxyCacheServer.processSocket(HttpProxyCacheServer.java:234)
at com.danikula.videocache.HttpProxyCacheServer.access$200(HttpProxyCacheServer.java:52) 
at com.danikula.videocache.HttpProxyCacheServer$SocketProcessorRunnable.run(HttpProxyCacheServer.java:338) 
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:423) 
at java.util.concurrent.FutureTask.run(FutureTask.java:237) 
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
at java.lang.Thread.run(Thread.java:818) 
Caused by: java.io.FileNotFoundException: xxx.mp4
at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:238)
at com.danikula.videocache.HttpUrlSource.open(HttpUrlSource.java:70)
at com.danikula.videocache.HttpProxyCache.responseWithoutCache(HttpProxyCache.java:89) 
at com.danikula.videocache.HttpProxyCache.processRequest(HttpProxyCache.java:47) 
at com.danikula.videocache.HttpProxyCacheServerClients.processRequest(HttpProxyCacheServerClients.java:42) 
at com.danikula.videocache.HttpProxyCacheServer.processSocket(HttpProxyCacheServer.java:234) 
at com.danikula.videocache.HttpProxyCacheServer.access$200(HttpProxyCacheServer.java:52) 
at com.danikula.videocache.HttpProxyCacheServer$SocketProcessorRunnable.run(HttpProxyCacheServer.java:338) 
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:423) 
at java.util.concurrent.FutureTask.run(FutureTask.java:237) 
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
at java.lang.Thread.run(Thread.java:818) 
```

Start with HttpProxyCacheServer.processSocket and the stack trace through HttpProxyCache.java and HttpUrlSource.java. Determine how a client could observe ProxyCacheException from the server, and verify that the proposed error notification behavior is consistent with the existing request-processing flow.

Written by the indexing model from the issue text.