Logformatter for errors

[mredaelli]

Summary

Allow users to specify the logging behaviour in case of errors in the item pipeline

Motivation

Looking at https://github.com/scrapy/scrapy/blob/3adf09bc4f1468f54a703481d6d7ebfe25b6f232/scrapy/core/scraper.py, it is possible to override the logging behaviour for the DropItem exception, but there is no equivalent
self.logformatter.error.

In our case, sometimes the item can be very large, in which case it being printed completely to the log is problematic -- specifically, on ScrapyingHub the log entry is refused and the exception is not visible.

Describe alternatives you've considered

Modify that file adding a generic error logformatter function.