snuffling: handle missing marker files in ifc

src/snuffling.py

 import copy
 import os
 import numpy as num
+import logging
+
 from pyrocko import util, model, orthodrome, pile
 from pyrocko.gui import snuffler
 from pyrocko.gui import marker as pmarker
@@ -9,6 +11,9 @@ from pyrocko.gui.snuffling import Snuffling, Param, Choice, Switch
 from lassie import geo, ifc
 
 
+logger = logging.getLogger('lassie.snuffling')
+
+
 kind_default = '1 (green)'
 
 
@@ -280,7 +285,11 @@ def snuffle(config):
             elif isinstance(_ifc, ifc.TemplateMatchingIFC):
                 markers_path_extra = _ifc.template_markers_path
 
-            s.add_markers(pmarker.load_markers(markers_path_extra))
+            if os.path.exists(markers_path_extra):
+                s.add_markers(pmarker.load_markers(markers_path_extra))
+            else:
+                logger.warn('No such file: %s (referenced in %s, named %s)' % (
+                    markers_path_extra, _ifc.__class__.__name__, _ifc.name))
 
     receivers = config.get_receivers()
     stations = set()