Handle configparser.InterpolationSyntaxError

Under Python 3, an option value with a single % (e.g. this % is a
test) throws

   configparser.InterpolationSyntaxError: '%' must be followed by
      '%' or '(', found: '%s))'

Added code to capture this, raise a different exception. roundup-admin
handles the error and exits cleanly. Other code shows the traceback.

The new error message reports the file, section and option causing the
problem to allow easier repair.

Also updated roundup translations and added tests.

Author: rouilj

CHANGES.txt

@@ -56,6 +56,9 @@ Fixed:
   config_ini.ini overrides. Needed for jinja to set template lang etc.
   Recognize minimal template when presented with a full
   path. (John Kristensen (jerrykan) and John Rouillard)
+- handle configparser.InterpolationSyntaxError raised if value
+  has a single %. Seems to afect python 3 only. Reported by
+  nomicon on IRC. (John Rouillard)
 
 Features: