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/d3baf4110e

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

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

https://github.com/rubygems/rubygems/commit/573ffad3ea

We now run specs against a single version, so I prefer to keep a single
branch. Once we bump the major version, this will need very little
updates, and that seems fine.

https://github.com/rubygems/rubygems/commit/3866d25a00

Running everything in `bundled_app` by default causes the `bundled_app`
helper to be used everytime, and that will create a scoped bundled_app
folder if it does not exist. That causes `bin/rake spec:deps` to create
an empty `tmp/2.1/bundled_app` folder which is a bit weird.

This commit changes specs to not switch to a (possibly empty)
bundled_app directory when not necessary (for example, when running
`gem` commands in order to setup test dependencies).

https://github.com/rubygems/rubygems/commit/4bf89c0705

Originally, all the specs in this file were put inside a shared examples
block, and since then all specs were run only changing the cwd (either
from root, or a subdirectory).

This was in https://github.com/rubygems/rubygems/commit/d7291700d016, to cover a fix in
the `bundler_path` method.

However, reverting that fix does not make any of the specs in either of
the main blocks fail! Only an unrelated spec of `bundle install
--standalone --local` fails.

The reason is that all specs set `path` to an absolute path, making the
fix essentially uncovered.

In order to simplify the file structure and improve runtime, I
completely removed the shared examples block, and only run main specs
for the root directory. Then I added a couple of extra specs to cover
the original bug fix.

This cuts runtime of this spec file in half, from 1m30s to 45s on my
laptop.

https://github.com/rubygems/rubygems/commit/cc506f17e0

Also removed the helper to install real gems during specs to avoid the
temptation of introducing network stuff again.

https://github.com/rubygems/rubygems/commit/a1ab5e319a