Improve error/warning reporting in `stack sdist` and `stack upload`

As discovered by @Blaisorblade in https://github.com/commercialhaskell/stack/issues/2501#issuecomment-240164466 the check reporting of stack sdist is much less informative than the analogous output from `cabal check`:

```
$ cabal check
The following warnings are likely affect your build negatively:
* The 'license-file' field refers to the file 'LICENSE' which does not exist.

Hackage would reject this package.
$ cabal sdist
Distribution quality errors:
The 'license-file' field refers to the file 'LICENSE' which does not exist.
Note: the public hackage server would reject this package.
Warning: Cannot run preprocessors. Run 'configure' command first.
Building source dist for foo-0.1.0.0...
LICENSE: copyFile: does not exist (No such file or directory)
$ stack sdist
Getting file list for /Users/pgiarrusso/AeroFS/Repos/stack-tatooine/foo/
Building sdist tarball for /Users/pgiarrusso/AeroFS/Repos/stack-tatooine/foo/
/Users/pgiarrusso/AeroFS/Repos/stack-tatooine/foo/LICENSE: getFileStatus: does not exist (No such file or directory)
```

> Specifically, stack sdist only says that LICENSE can't be found, not that it's mentioned in license-file (which was genuinely non-obvious in #2500), nor any of the extra info from cabal-install.