.tgz extension in extra-deps archive rejected with confusing error.

With this `stack.yaml`,

```yaml
resolver: lts-15.13
extra-deps:
- archive: /home/whatever/mypackage.tgz
sha256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
```

I get this:

```
$ stack --version
Version 2.4.0, Git revision 9dcef52902d01646d63fe76fc8e6b1b3ac6cc9b8 (dirty) (8118 commits) PRE-RELEASE x86_64 hpack-0.33.0
$ stack build
Could not parse '/test/stack.yaml':
Aeson exception:
Error in $['extra-deps'][0]: failed to parse field 'extra-deps': parsing Text failed, expected String, but encountered Object
See http://docs.haskellstack.org/en/stable/yaml_configuration/
```

1. `.tgz` is a standard abbreviation for `.tar.gz`, so it's somewhat unexpected that it's not accepted. (This is presumably a Pantry issue.)

2. That error message is not helpful or really accurate. Using a `.tar.gz` extension works, so that is the actual error. (Using `.tar.xz`, etc., yields the same error.) I see proper error text in the Pantry source, but it's not getting through.

3. Minor, but the doc URL should be updated to `https:`.