Improve staging website update error reports

Staging website update error reports are sent to the translation platform maintainers list. This specific report has been improved a lot lately, but still needs some fine tuning:

• Completelly remove the output of ikiwiki --refresh from e-mail reports. This is because (1) Ikiwiki is too verbose, and (2) whenever that step fails, a ikiwiki --rebuild is executed immediately, and that has less chances to fail and informative enough output in itself whenever needed.
• Make sure the command which caused the output is included in the reports.

See also the patch proposed by intrigeri in March 2020 on tails/tails#17503.