Improve MuxBearerClosed Message

[karknu]

Make MuxBearerClosed message less intimidating in case we're waiting on a new header. It is not the end of the world, a peer just closed their connection towards us.

[dcoutts]

Current output looks like:

{"event":"ErrorPolicyUnhandledApplicationException (MuxError {errorType = MuxBearerClosed,
 errorMsg = \"<socket: 29> closed when reading data, waiting on next header True\",
 errorStack = []})","kind":"ErrorPolicyTrace","address":"LocalAddress {getFilePath = \"\"}"}

[coot]

Related to #2524