IntervalUtils::loadIntervals gives bad error on missing .interval_list file

[ldgauthier]

A Picard-style .interval_list file is a feature file, but FeatureManager.isFeatureFile also checks its existence, which means a feature file path that is missing gets (unsuccessfully) parsed as an interval string literal and throws a misleading exception:
org.broadinstitute.hellbender.exceptions.UserException$MalformedGenomeLoc: Badly formed genome unclippedLoc: Query interval "missing_file.interval_list" is not valid for this input.

This should be a quick fix, I just don't want to add more to the PR I'm working on.