add informative prints to assert statements (#1360)

Summary:
Pull Request resolved: #1360

Adding information in print for asserts about metrics, to save user time debugging the assert

Reviewed By: m3rlin45

Differential Revision: D21528177

fbshipit-source-id: b1527278877cc5f914a327d64881e40cc9861e96

Author: rutyrinott

pytext/metric_reporters/classification_metric_reporter.py

@@ -102,13 +102,17 @@ def from_config_and_label_names(cls, config, label_names: List[str]):
             ComparableClassificationMetric.LABEL_AVG_PRECISION,
             ComparableClassificationMetric.LABEL_ROC_AUC,
         ):
-            assert config.target_label is not None
+            assert (
+                config.target_label is not None
+            ), "target_label must be set for selected metric"
             assert config.target_label in label_names
         if config.model_select_metric in (
             ComparableClassificationMetric.ROC_AUC,
             ComparableClassificationMetric.MCC,
         ):
-            assert len(label_names) == 2
+            assert (
+                len(label_names) == 2
+            ), "selected metric is valid for binary labels only"
 
         return cls(
             label_names,