Hotfix broken raise statement

[neutrinoceros]

PR Summary

fix a broken raise statement (missing positional argument in YTFieldNotFound init).
While I'm at it, I'm changing the pattern to EAFP.

edit:
Wait. This error is actually encapsulating another one that's pretty much identical

KeyError: 'No field named blblblb'
During handling of the above exception, another exception occurred:

YTFieldNotFound 
YTFieldNotFound: Could not find field 'blblblb' in MYFILE.

So I think the best course of action is simply to get rid of this redundant error-chaining.

But there's worse: I'm getting this while I'm trying to add a field to a dataset, so none of these errors makes much sense to me.
-> this is a different problem and is actually fixed with #2638

[matthewturk]

So I am trying to recall if there are places we catch YTFieldNotFound as a signal, but I can't find them. So this looks good to me.