pipeline-preferred event table returns error if em_bright.json includes non-number values

This error came from sentry and was the result of a user going to a superevent page on the web in which a pipeline-preferred event's em_bright.json file was uploaded with non-number values (in this case, a null). Example on gracedb-test.

When gracedb attempted to round the value, it threw up an error. There should be a catch if the user uploads a file that contains values that can't be rounded. This error is only on the web but it still shouldn't be there and isn't helpful in the case of debugging.