From impls for errors generally just take some error type and map
it into a variant of some podcast error enum. This removes the duplicate
impls by using a pattern macro to make the impls from the type of the
enum, the given error type, and the desired enum variant.
This is never used anywhere else apart from the testsuite. Instead
of ignoring etags we should instead not save them if the feed does
not return 200 or 304. See #64.
