replace logger.warning with warn_or_error

Uses `warn_or_error` to log _or_ raise errors depending on arguments
passed.

core/dbt/task/list.py

@@ -8,8 +8,8 @@
 from dbt.task.runnable import GraphRunnableTask, ManifestTask
 from dbt.task.test import TestSelector
 from dbt.node_types import NodeType
-from dbt.exceptions import RuntimeException, InternalException
-from dbt.logger import log_manager, GLOBAL_LOGGER as logger
+from dbt.exceptions import RuntimeException, InternalException, warn_or_error
+from dbt.logger import log_manager
 
 
 class ListTask(GraphRunnableTask):
@@ -61,7 +61,7 @@ def _iterate_selected_nodes(self):
         spec = self.get_selection_spec()
         nodes = sorted(selector.get_selected(spec))
         if not nodes:
-            logger.warning('No nodes selected!')
+            warn_or_error('No nodes selected!')
             return
         if self.manifest is None:
             raise InternalException(

core/dbt/task/runnable.py

@@ -39,6 +39,7 @@
     NotImplementedException,
     RuntimeException,
     FailFastException,
+    warn_or_error,
 )
 
 from dbt.graph import (
@@ -438,8 +439,8 @@ def run(self):
             )
 
         if len(self._flattened_nodes) == 0:
-            logger.warning("\nWARNING: Nothing to do. Try checking your model "
-                           "configs and model specification args")
+            warn_or_error("\nWARNING: Nothing to do. Try checking your model "
+                          "configs and model specification args")
             result = self.get_result(
                 results=[],
                 generated_at=datetime.utcnow(),