Raise explicit KeyError for copy job resource w/o source table(s).

Addresses:
googleapis/google-cloud-python#2884 (comment)

Author: tseaver

google/cloud/bigquery/job.py

@@ -744,7 +744,11 @@ def from_api_repr(cls, resource, client):
         sources = []
         source_configs = config.get('sourceTables')
         if source_configs is None:
-            source_configs = [config['sourceTable']]
+            single = config.get('sourceTable')
+            if single is None:
+                raise KeyError(
+                    "Resource missing 'sourceTables' / 'sourceTable'")
+            source_configs = [single]
         for source_config in source_configs:
             dataset = Dataset(source_config['datasetId'], client)
             sources.append(Table(source_config['tableId'], dataset))

unit_tests/test_job.py

@@ -795,6 +795,29 @@ def test_from_api_repr_w_sourcetable(self):
         self.assertIs(job._client, client)
         self._verifyResourceProperties(job, RESOURCE)
 
+    def test_from_api_repr_wo_sources(self):
+        self._setUpConstants()
+        client = _Client(self.PROJECT)
+        RESOURCE = {
+            'id': self.JOB_ID,
+            'jobReference': {
+                'projectId': self.PROJECT,
+                'jobId': self.JOB_NAME,
+            },
+            'configuration': {
+                'copy': {
+                    'destinationTable': {
+                        'projectId': self.PROJECT,
+                        'datasetId': self.DS_NAME,
+                        'tableId': self.DESTINATION_TABLE,
+                    },
+                }
+            },
+        }
+        klass = self._get_target_class()
+        with self.assertRaises(KeyError):
+            klass.from_api_repr(RESOURCE, client=client)
+
     def test_from_api_repr_w_properties(self):
         client = _Client(self.PROJECT)
         RESOURCE = self._makeResource()