Fix logging of AsyncSocket connection failed exceptions

Summary:
When log verbosity is sufficiently high, the logging of AsyncSocket connection exceptions triggers the following assert:
```
  assert(typeid(ex) == typeid(std::decay_t<Ex>) ||
       !"Dynamic and static exception types don't match. Exception would "
        "be sliced when storing in exception_wrapper.");
```
Update the log to output `e.what()`, which all of the other exception logs already do.

Reviewed By: yfeldblum

Differential Revision: D26799863

fbshipit-source-id: f9d98f81a77a654e78a153bc5c23e6ec8223fa1f

folly/io/async/AsyncSocket.cpp

@@ -3400,7 +3400,7 @@ void AsyncSocket::invalidState(ErrMessageCallback* callback) {
 
 void AsyncSocket::invokeConnectErr(const AsyncSocketException& ex) {
   VLOG(5) << "AsyncSocket(this=" << this << ", fd=" << fd_
-          << "): connect err invoked with ex: " << ex;
+          << "): connect err invoked with ex: " << ex.what();
   connectEndTime_ = std::chrono::steady_clock::now();
   if (connectCallback_) {
     ConnectCallback* callback = connectCallback_;