_context: refactor, extract _node_get_option_str

Use a new helper function to simplify working with nodes that can only
accept certain strings. This will be used when adding the prompt.*
config options.

In later work we can see if this function would be useful elsewhere, and
could be added to '_yaml.py'.

buildstream/_context.py

@@ -206,7 +206,8 @@ class Context():
             'on-error', 'fetchers', 'builders',
             'pushers', 'network-retries'
         ])
-        self.sched_error_action = _yaml.node_get(scheduler, str, 'on-error')
+        self.sched_error_action = _node_get_option_str(
+            scheduler, 'on-error', ['continue', 'quit', 'terminate'])
         self.sched_fetchers = _yaml.node_get(scheduler, int, 'fetchers')
         self.sched_builders = _yaml.node_get(scheduler, int, 'builders')
         self.sched_pushers = _yaml.node_get(scheduler, int, 'pushers')
@@ -222,13 +223,6 @@ class Context():
 
         profile_end(Topics.LOAD_CONTEXT, 'load')
 
-        valid_actions = ['continue', 'quit', 'terminate']
-        if self.sched_error_action not in valid_actions:
-            provenance = _yaml.node_get_provenance(scheduler, 'on-error')
-            raise LoadError(LoadErrorReason.INVALID_DATA,
-                            "{}: on-error should be one of: {}".format(
-                                provenance, ", ".join(valid_actions)))
-
     @property
     def artifactcache(self):
         if not self._artifactcache:
@@ -581,3 +575,30 @@ class Context():
             os.environ['XDG_CONFIG_HOME'] = os.path.expanduser('~/.config')
         if not os.environ.get('XDG_DATA_HOME'):
             os.environ['XDG_DATA_HOME'] = os.path.expanduser('~/.local/share')
+
+
+# _node_get_option_str()
+#
+# Fetches a value from a dictionary node, and makes sure it's one of the
+# pre-defined options.
+#
+# Args:
+#    node (dict): The dictionary node
+#    key (str): The key to get a value for in node
+#    allowed_options (iterable): Only accept these values
+#
+# Returns:
+#    The value if found in node
+#
+# Raises:
+#    LoadError, when the value found is not of the expected type, or is not
+#    found.
+#
+def _node_get_option_str(node, key, allowed_options):
+    result = _yaml.node_get(node, str, key)
+    if result not in allowed_options:
+        provenance = _yaml.node_get_provenance(node, key)
+        raise LoadError(LoadErrorReason.INVALID_DATA,
+                        "{}: {} should be one of: {}".format(
+                            provenance, key, ", ".join(allowed_options)))
+    return result