QNAM HTTP: Fix missing error() signal

QNAM HTTP: Fix missing error() signal

During download, if a QAbstractSocket::RemoteHostClosedError occurs, the error handling is deferred to _q_disconnected() slot. However, the error message is not saved for the function, and thus the _q_disconnected only emits a finished-signal.
We now store an unhandled error into a private member. It is handled and reset by the _q_disconnected (or more specifically, allDone-function). Also, an additional check is made that there are no simultaneous HTTP errors. If a HTTP error exists, any socket errors are suppressed.

Task-Number: QT-3494
Reviewed-By: Markus Goetz