This error message was very confusing -- it doesn't tell you what
`.cabal` file it's looking for or why it cannot read the `.cabal` file.
(Did it fail to parse the `.tar` archive itself? Did parsing the
`.cabal` file fail? If so, why?)

This patch improves the error message to include the name of the
`.cabal` file being searched for. Additionally, parse errors and
warnings are printed, as are format failures in the tarball itself.

I ran into this error while I was writing tests for Cabal and it
confused the hell out of me; this is an expanded version of the changes
I made to help debug that failure.

Before:

```
Error: [Cabal-7046]
Cannot read .cabal file inside my-lib-1.0.tar.gz
```

After:

```
Error: In <ROOT>/cabal.dist/repo/my-lib-1.0.tar.gz: Errors encountered when parsing cabal file my-lib-1.0/my-lib.cabal:

my-lib-1.0/my-lib.cabal:4:22: error:
unexpected Unknown SPDX license identifier: 'puppy'

    3 | version:        1.0
    4 | license:        puppy license :)
      |                      ^

my-lib-1.0/my-lib.cabal:5:1: warning:
Unknown field: "puppy"

    4 | license:        puppy license :)
    5 | puppy:          teehee!
      | ^
Error: [Cabal-7046]
Failed to read my-lib-1.0/my-lib.cabal from archive <ROOT>/cabal.dist/repo/my-lib-1.0.tar.gz
```