Better error message on license check

After today's Hackage update, I get

```
% cabal upload dist/swish-0.9.2.1.tar.gz
Hackage username: NotMyUserName
Hackage password: DefinitelyNotMyPassword
Uploading dist/swish-0.9.2.1.tar.gz...
Error uploading dist/swish-0.9.2.1.tar.gz: http code 400
Error: Invalid package

This server does not accept packages with 'license' field set to e.g.
AllRightsReserved. See https://hackage.haskell.org/upload for more information
about accepted licenses.
```

which I don't understand since

```
% grep -i license swish.cabal
License: LGPL
License-file: LICENSE
```

and Hackage was perfectly fine with this license on March 8th when I last uploaded a version (https://hackage.haskell.org/package/swish-0.9.2.0).

For reference the cabal file used when attempting the upload is https://bitbucket.org/doug_burke/swish/src/6d1b4d4fad760b64904dff6230a1612464cf9b24/swish.cabal?at=default&fileviewer=file-view-default

and

```
% cabal --version
cabal-install version 1.24.0.2
compiled using version 1.24.2.0 of the Cabal library
```