Attempting to set ROOT error level around file opening in EL

Closed Zach Marshall requested to merge zmarshal/athena:EL_WorkerErrorLevel into master

We have this notorious problem that ROOT doesn't always communicate error messages in a reasonable way when opening a file fails. One of the only ways to catch them is to set ROOT to abort if an error message is printed to the log.

This attempts to set that abort level around the time that the file is opened in event loop, which should protect us against those errors (if it works).

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