Convert two common plugin errors from unhandled exceptions to warnings.

Missing translation for the current primary locale.
Import failure because of some missing dependency.

Note that these handlers can be overridded in the module itself; this is just a fallback.

gramps/gen/plug/_manager.py

@@ -41,6 +41,9 @@ from __future__ import print_function
 import os
 import sys
 import re
+import logging
+LOG = logging.getLogger('.' + __name__)
+LOG.progagate = True
 from ..const import GRAMPS_LOCALE as glocale
 _ = glocale.translation.gettext
 
@@ -252,7 +255,12 @@ class BasePluginManager(object):
         if pdata.fpath not in sys.path:
             if pdata.mod_name:
                 sys.path.insert(0, pdata.fpath)
-                module = __import__(pdata.mod_name)
+                try:
+                    module = __import__(pdata.mod_name)
+                except ValueError as err:
+                    LOG.warning('Plugin error: %s', err)
+                except ImportError as err:
+                    LOG.warning('Plugin error: %s', err)
                 sys.path.pop(0)
             else:
                 print("WARNING: module cannot be loaded")