ReblockGVCFs is slow when --tree-score-threshold-to-no-call is used without TREE_SCORE annotation

[meganshand]

Based on this forum post, when the argument --tree-score-threshold-to-no-call is used in ReblockGVCFs with input that doesn't contain the TREE_SCORE annotation, the tool gets very slow, since it tries to no-call every variant. An error message should be thrown before starting traversal of the VCF if the input is missing this annotation when that argument is set.