Don’t swallow virtual manifest parsing errors

Don’t swallow virtual manifest parsing errors

Before this change, `cargo::util::toml::do_read_manifest` ended like this:

```rust
    return match TomlManifest::to_real_manifest(/* … */) {
        Ok(/* … */) => /* … */,
        Err(e) => match TomlManifest::to_virtual_manifest(/* … */) {
            Ok(/* … */) => /* … */,
            Err(..) => Err(e),
        }
    };
```

Errors returned by `to_virtual_manifest` were always ignored.
As a result, when something was wrong in a virtual manifest,
Cargo would unhelpfully exit with no more output than:

```
error: failed to parse manifest at `/tmp/a/Cargo.toml`

Caused by:
  no `package` section found.
```

http://doc.crates.io/manifest.html#virtual-manifest defines
a virtual manifest as “the `package` table is not present”,
so let’s first determine if a manifest is virtual based
on that criteria, and then only call one of the two methods.

Although it is not mentioned in the documentation,
`[project]` seems to be in the code an alias for `[package]`.
So let’s preserve that here too.