- if the queue is emptied, there was no usable data; set
failure to NoData rather than BadData.
- FetchNextUnless::done() is done only if the parameter is true (that
is, it's done!); otherwise should continue.
- use load() to start loading
- the FetchNextUnless class is useful in more spots in
the loading process
- set status explicitly on success (otherwise, a failure in a
previous URL would leave a failure message lying around even
when the module shows something useful)
- m_queue was not initialized to nullptr, crashes
- split queue-is-done to a separate slot rather than a lambda
- prefer queueing calls to fetchNext(), for responsiveness
