Avoid a NPE if the public URL is not set.

--HG--
extra : rebase_source : 9b98f856d10dff6cb19bbaf90fb7eb22336aea7f
Make subscription failures visible.

--HG--
extra : rebase_source : 9e7232b821665d9a6342c928d0b17ac3bdd9e6e7
Make the error message prettier by avoiding a nested exception.

--HG--
extra : rebase_source : 8c33ce8a8a6ae92fcc1e46a7b416e1c9802bf726