Generate an error when _events.tsv is present in func without corresponding bold data

[robertoostenveld]

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Some examples were identified in https://github.com/bids-standard/bids-examples/pull/166 that have an _events.tsv file without the corresponding _bold.ni.gz. This was apparently not detected by the validator. Presumably the same problem applies to _events.tsv in other modalities (except beh, where it is appropriate), and presumably also for other files that accompany data, such as channels, electrodes, headshape, coordsys, etc.