This is a first pass to improve the way errors are handled and raised in
bundler's tests. The goal is to clean up tests and modernize them -
these were some obvious areas that could be cleaned up.

- Instead of raising "ZOMG" in the load error tests, it now tests for
the actual error and gem raising.
- Improve error messages where applicable.
- All errors raise a specific error class, rather than falling back to a
default and just setting a message.
- Removed arguments and `bundle_dir` option from `TheBundle` class as it wasn't
actually used so therefore we don't need to raise an error for extra
arguments.
- Removed error from `BundlerBuilder`, as it won't work if it's not
`bundler`, also it never uses `name`. The only reaon `name` is passed
in is because of metaprogramming on loading the right builder. I
think that should eventually be refactored.
- Replaced and removed `update_repo3` and `update_repo4` in favor of
just `build_repo3` and `build_repo4`. Rather than tell someone writing
tests to use a different method, automatically use the right method.

https://github.com/ruby/rubygems/commit/68c39c8451

```
$ rake spec:regular
  Successfully built RubyGem
  Name: bundler
  Version: 4.0.0.dev
  File: bundler-4.0.0.dev.gem
```

https://github.com/ruby/rubygems/commit/8f0ca5eefa

This is a follow up to #9053. We can avoid shelling out for generating
the gem index.

https://github.com/ruby/rubygems/commit/e40bafe9f1

https://github.com/ruby/rubygems/commit/2142e405b0

I'm trying to speed up the bundler tests. The tests shell out a lot in
order to build gems. We can build gems without creating a sub-process.
This change reduced the test suite time from ~24 minutes, to about ~21
minutes on my machine.  Once we have more of these "asset generation"
routines done in the same process, I think we can start caching the
outputs for further improvements

https://github.com/ruby/rubygems/commit/ebf27056c6

https://github.com/ruby/rubygems/commit/33c7a9a565

https://github.com/rubygems/rubygems/commit/3189d3e49b