Pass errors from MITM SSL sockets up to the proxy object

[mbell697]

This PR passes socket errors from the SSL MITM code path up to the the top level Proxy object so they may be handled externally.

Without this fix, I was seeing ECONNRESET errors bubble up as uncaught errors which could only be handled with a process.on('uncaughtException', .....) block. I believe the source of these errors is random client disconnects when using hoxy in a cross browser testing environment via selenium.

I didn't see an obvious way to introduce tests for this in the current test suite, if I've missed something here please point me in the right direction and I'll add them.