Werk #15606: Show explanation message instead of crashing

Previously, if the DCD tried to contact a remote site, then it would show the error

  File "/omd/sites/heute/lib/python3/cmk/cee/dcd/connectors/utils.py", line 221, in _execute_sync
    phase1_result = self._fetch_phase1_result_from_remote_site()
  File "/omd/sites/heute/lib/python3/cmk/cee/dcd/connectors/utils.py", line 259, in _fetch_phase1_result_from_remote_site
    phase_one_result = self._web_api.execute_phase1_result(
  File "/omd/sites/heute/lib/python3/cmk/cee/dcd/web_api.py", line 303, in execute_phase1_result
    return result.json()["extensions"]
KeyError: 'extensions'

With this Werk, the message Failed to contact remote site will be shown instead. The remaining phases were working as expected.

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