env-variables: make it safer if there's a syntax problem inside .ini file.

It's better to not crash, since it means server wont start. Let users fix problems
instead of breaking the startup because of that.

rhodecode/config/middleware.py

@@ -578,5 +578,8 b' def _substitute_values(mapping, substitu'
         raise ValueError(
             'Failed to substitute env variable: {}. '
             'Make sure you have specified this env variable without ENV_ prefix'.format(e))
+    except ValueError as e:
+        log.warning('Failed to substitute ENV variable: %s', e)
+        result = mapping
 
     return result