Exception while starting flashback proxy

while doing ```curl "http://localhost:1234/admin?action=startFlashback" --data '{
"sceneMode": "record",
"sceneName": "test1",
"matchRule": "matchEntireRequest",
"scenePath": "/tmp",
"proxyHost": "localhost",
"proxyPort": "5555"
}'```

we get:

```{"exceptionClass":"com.linkedin.restli.server.RestLiServiceException","message":"INTERNAL SERVER ERROR","stackTrace":"com.linkedin.restli.server.RestLiServiceException [HTTP Status:500]: INTERNAL SERVER ERROR\n\tat com.linkedin.restli.internal.server.RestLiMethodInvoker.doInvoke(RestLiMethodInvoker.java:317)\n\tat com.linkedin.restli.internal.server.RestLiMethodInvoker.access$100(RestLiMethodInvoker.java:62)\n\tat com.linkedin.restli.internal.server.RestLiMethodInvoker$RestLiRequestFilterChainCallbackImpl.onSuccess(RestLiMethodInvoker.java:424)\n\tat com.linkedin.restli.internal.server.filter.RestLiRequestFilterChain.onRequest(RestLiRequestFilterChain.java:72)\n\tat com.linkedin.restli.internal.server.RestLiMethodInvoker.invoke(RestLiMethodInvoker.java:198)\n\tat com.linkedin.restli.server.RestLiServer.handleResourceRequest(RestLiServer.java:337)\n\tat com.linkedin.restli.server.RestLiServer.doHandleRequest(RestLiServer.java:227)\n\tat com.linkedin.restli.server.BaseRestServer.handleRequest(BaseRestServer.java:61)\n\tat com.linkedin.restli.server.DelegatingTransportDispatcher.handleRestRequest(DelegatingTransportDispatcher.java:61)\n\tat com.linkedin.r2.filter.transport.DispatcherRequestFilter.onRestRequest(DispatcherRequestFilter.java:68)\n\tat com.linkedin.r2.filter.FilterChainIterator$FilterChainRestIterator.doOnRequest(FilterChainIterator.java:131)\n\tat com.linkedin.r2.filter.FilterChainIterator$FilterChainRestIterator.doOnRequest(FilterChainIterator.java:117)\n\tat com.linkedin.r2.filter.FilterChainIterator.onRequest(FilterChainIterator.java:57)\n\tat com.linkedin.r2.filter.transport.ServerQueryTunnelFilter.onRestRequest(ServerQueryTunnelFilter.java:58)\n\tat com.linkedin.r2.filter.FilterChainIterator$FilterChainRestIterator.doOnRequest(FilterChainIterator.java:131)\n\tat com.linkedin.r2.filter.FilterChainIterator$FilterChainRestIterator.doOnRequest(FilterChainIterator.java:117)\n\tat com.linkedin.r2.filter.FilterChainIterator.onRequest(FilterChainIterator.java:57)\n\tat com.linkedin.r2.filter.message.rest.RestFilter.onRestRequest(RestFilter.java:50)\n\tat com.linkedin.r2.filter.FilterChainIterator$FilterChainRestIterator.doOnRequest(FilterChainIterator.java:131)\n\tat com.linkedin.r2.filter.FilterChainIterator$FilterChainRestIterator.doOnRequest(FilterChainIterator.java:117)\n\tat com.linkedin.r2.filter.FilterChainIterator.onRequest(FilterChainIterator.java:57)\n\tat com.linkedin.r2.filter.FilterChainImpl.onRestRequest(FilterChainImpl.java:86)\n\tat com.linkedin.r2.filter.transport.FilterChainDispatcher.handleRestRequest(FilterChainDispatcher.java:70)\n\tat com.linkedin.r2.transport.http.server.HttpDispatcher.handleRequest(HttpDispatcher.java:95)\n\tat com.linkedin.r2.transport.http.server.HttpDispatcher.handleRequest(HttpDispatcher.java:70)\n\tat com.linkedin.r2.transport.http.server.HttpNettyServer$RestHandler.channelRead0(HttpNettyServer.java:173)\n\tat com.linkedin.r2.transport.http.server.HttpNettyServer$RestHandler.channelRead0(HttpNettyServer.java:136)\n\tat io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)\n\tat io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)\n\tat io.netty.channel.AbstractChannelHandlerContext.access$600(AbstractChannelHandlerContext.java:32)\n\tat io.netty.channel.AbstractChannelHandlerContext$7.run(AbstractChannelHandlerContext.java:329)\n\tat io.netty.util.concurrent.DefaultEventExecutor.run(DefaultEventExecutor.java:36)\n\tat io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)\n\tat io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)\n\tat java.lang.Thread.run(Thread.java:748)\nCaused by: io.netty.channel.ChannelException: Failed to bind to: localhost:5555\n\tat com.linkedin.mitm.proxy.ProxyServer.start(ProxyServer.java:87)\n\tat com.linkedin.flashback.smartproxy.FlashbackRunner.start(FlashbackRunner.java:99)\n\tat com.linkedin.flashback.FlashbackAdminResource.startFlashback(FlashbackAdminRe* Curl_http_done: called premature == 0Connection #0 to host localhost left intact source.java:80tat sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)\n\tat sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)\n\tat sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)\n\tat java.lang.reflect.Method.invoke(Method.java:498)\n\tat com.linkedin.restli.internal.server.RestLiMethodInvoker.doInvoke(RestLiMethodInvoker.java:243)\n\t... 34 more\nCaused by: java.net.BindException: Address already in use\n\tat sun.nio.ch.Net.bind0(Native Method)\n\tat sun.nio.ch.Net.bind(Net.java:433)\n\tat sun.nio.ch.Net.bind(Net.java:425)\n\tat sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:223)\n\tat sun.nio.ch.ServerSocketAdaptor.bind(ServerSocketAdaptor.java:74)\n\tat io.netty.channel.socket.nio.NioServerSocketChannel.doBind(NioServerSocketChannel.java:125)\n\tat io.netty.channel.AbstractChannel$AbstractUnsafe.bind(AbstractChannel.java:485)\n\tat io.netty.channel.DefaultChannelPipeline$HeadContext.bind(DefaultChannelPipeline.java:1081)\n\tat io.netty.channel.AbstractChannelHandlerContext.invokeBind(AbstractChannelHandlerContext.java:502)\n\tat io.netty.channel.AbstractChannelHandlerContext.bind(AbstractChannelHandlerContext.java:487)\n\tat io.netty.channel.DefaultChannelPipeline.bind(DefaultChannelPipeline.java:904)\n\tat io.netty.channel.AbstractChannel.bind(AbstractChannel.java:198)\n\tat io.netty.bootstrap.AbstractBootstrap$2.run(AbstractBootstrap.java:348)\n\tat io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:357)\n\tat io.netty.channel.nio.NioEventLoop.run(NioEventLo```

Start at FlashbackAdminResource.startFlashback, then follow FlashbackRunner.start and ProxyServer.start in the stack trace. Reproduce the curl request with port 5555 occupied and determine the intended behavior for the reported bind failure; the issue is done only when that behavior is specified and verified.

Written by the indexing model from the issue text.