condor_history_to_elasticsearch.py: parse "Err" file and store error messages

[gonzalomerino]

I think this could be useful.

As we parse the condor_history with this tool, for inserting it to ES, we can look inside the error file pointed to by the classAdd "Err" and store some lines from there that can contain useful error messages for categorizing the failures.

For example, gridftp "get" errors from FileStager inside IceTray will look like this:

RuntimeError: globus-url-copy failed with status 1: error: globus_ftp_client: the operation was aborted (in CopyFileIn)

having this info in ES sounds like a useful info to me for doing error categorization and posterior analysis.