confluence: correctly report a failed pdf-export

[MaZderMind]

The Code in get_pdf_download_url_for_confluence_cloud checks for errors in the PDF Export like this:

                if percentage_complete == 100:
                    running_task = False
                    log.info("Task completed - {task_state}".format(task_state=task_state))
                    if task_state == "FAILED":
                        log.error("PDF conversion not successful.")
                        return None
                    log.debug("Extract task results to download PDF.")
                    task_result_url = progress_response.get("result")
                else:
…

However, pages can fail at any percentage, leading to repeated checks (which never complete):

INFO:atlassian.confluence:50% - FAILED
INFO:atlassian.confluence:Check if export task has completed.
INFO:atlassian.confluence:50% - FAILED
INFO:atlassian.confluence:Check if export task has completed.
…

This change makes the code check the Task-State at every poll-roundtrip and return an error whenever it is ERROR.
fixes #1202

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (5d9cd15) 34.84% compared to head (96f82b5) 34.84%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1203   +/-   ##
=======================================
  Coverage   34.84%   34.84%           
=======================================
  Files          44       44           
  Lines        7940     7940           
  Branches     1072     1072           
=======================================
  Hits         2767     2767           
  Misses       5061     5061           
  Partials      112      112           

Impacted Files	Coverage Δ
atlassian/confluence.py	13.16% <0.00%> (ø)

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