better mako diagnostic when missing a symbol (when importing)

better mako diagnostic when missing a symbol ( when doing nikola import_zzz )

From mako docs:
"""
strict_undefined – 

Replaces the automatic usage of UNDEFINED for any undeclared variables not located in the Context with an immediate raise of NameError. The advantage is immediate reporting of missing variables which include the name.
"""

Tested with nikola import_blogger , works as intended.