[Components] Simple 404 Handling (Fallback Route)

[TheFanatr]

This is a migration of Blazor pull request 1534. I have talked about it there and it has been reviewed. I simply applied the same changes to the refactored sources in this repo. Essentially, it enables simple error 404 handling that causes another (fallback) component to render if no components were found for a requested route. Please visit the aforementioned link for more details.

[TheFanatr]

The CI build seems to have failed for unrelated reasons as far as I can tell.

[SteveSandersonMS]

Moving to #4794, so I can make a further tweak to change the parameter to be the actual component type, rather than a route string. This is to make it more strongly-typed.

[TheFanatr]

I agree that strong-typing is good in most cases, and recognize that this has already been merged now, but I'd still like to make the argument for a route string in this case. Being able to define a fallback route instead of just a component allows for the specification of parameters as well as a component type implicitly, meaning that in theory, a generic error page could simply be created with a parameter for error type, which would be set to 404 if used as a fallback route, or any other error if routed to by another component. I realize that this may not actually be that common a scenario, and that most people would prefer to just have a 404 component, but I think it is worth mentioning. Also, keeping it as a simple route allows for the functionality to be expanded into supporting routing to an MVC page, a static asset, or even another website as a fallback, though this wasn't supported initially anyways. Either way, thanks for merging.

[SteveSandersonMS]

You can still do all that. The fallback component can read the URL and use any logic to act on it (including rendering another component or performing a full page reload to a server-side endpoint).