wrap filters/conditionals in a try-except block

allows accessing undefined fields without exception or locals().get(…)
but hides mistakes/typos/etc by evaluating to False without feedback

performance loss compared to the previous version without try-except
is negligible (~20ns for me)

gallery_dl/__init__.py

@@ -105,6 +105,11 @@ def main():
 
             output.ANSI = True
 
+        # filter environment
+        filterenv = config.get((), "filters-environment", True)
+        if not filterenv:
+            util.compile_expression = util.compile_expression_raw
+
         # format string separator
         separator = config.get((), "format-separator")
         if separator: