[dataimport] Have ucsvreader's API match that of csv.reader (closes #3705701) 'separator' and 'quote' are replaced with 'delimiter' and 'quotechar'.

[utils] Add a '_cwtracehtml' GET parameter to trace self._cw.w() calls (closes #4601327) The core of this patch is in UStringIO.write(). When tracing is enabled, write() doesn't just append the 'value' argument to the underlying list. Instead, a stack trace is recorded and a special HTML "source" is formatted. The output with tracing enabled is an HTML page, with the original HTML escaped, and made clickable to show the stack trace when the write() call was done. This allows answering the recurring question: "who wrote this tag here?!"

[datafeed] Add a raise_on_error parameter to DataFeedSource.extid2entity And pass the option from various `process_` methods in existing parsers. This makes debugging in tests easier. Closes #4601191.