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

https://github.com/ruby/rubygems/commit/1dc669a0ab

https://github.com/ruby/rubygems/commit/d3baf4110e

https://github.com/rubygems/rubygems/commit/9d70887185

https://github.com/rubygems/rubygems/commit/8f9d6c54a1

https://github.com/rubygems/rubygems/commit/c903afbc3f

https://github.com/rubygems/rubygems/commit/d41b8d303c

If upgrading `foo` needs an indirect dependency to be downgraded,
Bundler would not be able to upgrade foo.

This is because when calculating the latest resolvable version of foo,
Bundler was still adding lower bound requirements on the locked versions
of all dependencies to avoid downgrades, effectively pinning foo to a
version older than the latest.

To fix this, instead of creating a second "unlocked" definition to
figure out the latest resolvable version, create a second unlocked
resolver, and DO NOT add lower bound requirements to it.

https://github.com/rubygems/rubygems/commit/00cc0ecc69

Since now every functionality that changes in Bundler 4 is under a
setting, we can enable that setting to test the new functionality,
without having to run our full CI twice.

This can actually be seen as increasing coverage, because Bundler 4
functionality will now be tested on Windows, MacOS, or any other
environment where previously "Bundler 4 mode" was not running.

https://github.com/rubygems/rubygems/commit/1cb3e009fc

https://github.com/rubygems/rubygems/commit/07762f6c1f