Publishing silently fails when non-writable gateway is used

As noted in https://github.com/ipfs/ipfs-companion/issues/285:

Loading the site (which utilizes writable gateway) with ipfs-companion browser extension is problematic:

  1. http://ipfessay.stavros.io redirects to https://www.eternum.io/ipns/QmSWnBwMKZ28tcgMFdihD8XS7p6QzdRSGf71cCybaETSsU/

  2. ipfs-companion redirects that to my local gateway http://127.0.0.1:8080/ipns/QmSWnBwMKZ28tcgMFdihD8XS7p6QzdRSGf71cCybaETSsU/

  3. Clicking on publish does nothing, because my local gateway is read-only and rejects HTTP PUT: 2017-09-29-180102_995x29_scrot

I think IPFessay should at least display error informing user that used gateway is not writable, and instructions on how to make it writable.