Define and add InsufficientDataExceptions to tree/sub models.
Define and add InsufficientDataExceptions to tree/sub models.

This adds a new exception class to be used when there is insufficient data to train the titer or substitution models. This commit also replaces previous warning print statements with this exception in cases where there is not enough data (i.e. less than 1 titer measurement)
nextstrain/augur,blab/nextstrain-augur,nextstrain/augur,nextstrain/augur