[1.7.x] Fixed #22412 -- More nuanced advice re template filters and exceptions.

Thanks Tim for review.

Backport of 7e3834ad from master

docs/howto/custom-template-tags.txt

@@ -88,9 +88,11 @@ Custom filters are just Python functions that take one or two arguments:
 For example, in the filter ``{{ var|foo:"bar" }}``, the filter ``foo`` would be
 passed the variable ``var`` and the argument ``"bar"``.
 
-Filter functions should always return something. They shouldn't raise
-exceptions. They should fail silently. In case of error, they should return
-either the original input or an empty string -- whichever makes more sense.
+Usually any exception raised from a template filter will be exposed as a server
+error. Thus, filter functions should avoid raising exceptions if there is a
+reasonable fallback value to return. In case of input that represents a clear
+bug in a template, raising an exception may still be better than silent failure
+which hides the bug.
 
 Here's an example filter definition: