Please include support for a way to .tap only errors

[hcldan]

I would love to have cleanup functions that would not require me to rethrow the error.

promise.tapcatch(function(error) {
   // clean up some stuff in scope
});

instead of

promise.catch(function(error) {
   // clean up some stuff in scope
   throw error;
});

low priority... would be nice to have.

[phpnode]

dupe of #589

[hcldan]

Which is a dupe of #517... so I guess this isn't going in, huh?

[benjamingr]

As others have pointed out, this is a dupe.