Allow FormatError to take either String or Gem for source

Most of the calls to `FormatError.new` pass `@gem` for the second argument, which has a `path` method.

But in one case—on package.rb:691 in `verify_gz`, the `source` argument is a `String`.

So if there's ever a GZip decode error when attempting to read the contents of the `data.tar.gz` file, instead of reporting the underlying GZip error (which might be something like "unexpected end of file"), we would report instead a NoMethodError coming from package.rb

```
Exception while verifying sorbet-0.5.11301.gem
ERROR:  While executing gem ... (NoMethodError)
    undefined method `path' for "data.tar.gz":String

        @path = source.path
                      ^^^^^
```

There are two ways to fix this:

1. Make `FormatError#initialize` aware of the fact that `source` might sometimes be a `String`
2. Make the call to `FormatError.new` in `verify_gz` pass `@gem` instead of `entry.full_name`.

I've chosen 1 because I think it's more useful to see "unexpected end of file in data.tar.gz" instead of "unexpected end of file in sorbet-0.5.11301.gem." The end of file **is actually** in data.tar.gz, not in the gem file itself, which was decoded successfully.

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