[datafeed] Complete the import log even if parser could not be found It happens that if the parser could not be found, _pull_data() would just return an empty dict without taking care to close the import log which it just opened. This leads to misleading information in the user interface where CWDataImport entities kept accumulating in the "imports" tab of CWSource primary view without anything else happening. So: * log an error message when parser cannot be found * always close (write logs and set "end_timestamp" attribute) import log when leaving _pull_data(). Closes #15505460.

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