Lower final switch default error to object.d template

[somzzz]

Druntime changes: dlang/druntime#1971

I still had to use SwitchErrorStatement because of the checks and actions the compiler performs for ErrorStatemenets vs regular Statements.

[dlang-bot]

Thanks for your pull request, @somzzz! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.

Some tips to help speed things up:

• smaller, focused PRs are easier to review than big ones

• try not to mix up refactoring or style changes with bug fixes or feature enhancements

• provide helpful commit messages explaining the rationale behind each change

Bear in mind that large or tricky changes may require multiple rounds of review and revision.

Please see CONTRIBUTING.md for more information.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[wilzbach]

Nits

[wilzbach]

https://issues.dlang.org/show_bug.cgi?id=9395 would be very useful here ;-)

[wilzbach]

It's a good style to provide an informative error message or do sth. like

assert(s.exp !is null, "SwitchErrorStatement needs to have a valid Expression.") 

and skip the if entirely.

[JinShil]

It's a good style to provide an informative error message or do sth. like...

@WalterBright disagrees: #7079 (comment)

Some people like assert messages, I don't. They bloat up the compiler. Leaving the message as a comment in the source code is plenty good enough.

I really don't mind either way, just an FYI.