Improve initialization for extra arches

[akx]

This PR is related to AUTOMATIC1111/stable-diffusion-webui#16144, where we have to do some contortions to install extra arches when Spandrel is late-imported.

It

1. makes the ValueError for duplicate architectures a specific exception type, so it can be caught...
2. ... by the new spandrel_extra_arches.install() convenience function, which will return False (and not raise) in case the main registry already has (any of) the arches it tries to wire up.

[RunDevelopment]

Sorry for the delay.

There are 2 things I want to talk about:

1. I really like the install function, but I think its API is a little weird. It only returns False for a single type of failure and throws otherwise. This also contradicts the documentation of the function.

   I think a better API would be def install(*, ignore_duplicates=False) -> None. Basically, it throws by default, but you can use install(ignore_duplicates=True) to ensure that the second install is a noop.

   Implementation-wise, we just have to add a ignore_duplicates=False argument to ArchRegistry#add, and then install can be implemented as simply:

   def install(*, ignore_duplicates: bool = False) -> None:
       """TODO: Add docs"""
       MAIN_REGISTRY.add(*EXTRA_REGISTRY, ignore_duplicates=ignore_duplicates)

2. Throwing a DuplicateArchitectureError is a breaking change. I don't think it's a bad change, but this is something we have to consider, especially since the need for a unique error has been diminished with the ignore_duplicates parameter. Edit: my bad

[akx]

Throwing a DuplicateArchitectureError is a breaking change.

It's not, IMO, unless you're catching things in a particularly weird way (like, if type(exc) is ValueError weird). Unless I'm missing something?

We used to throw ValueError, now we throw DuplicateArchitectureError which derives from ValueError, so if you have a except ValueError, the new exception would still be caught too.

I'll get back to the other concerns later, work-work is pressing.

[RunDevelopment]

It's not [...]. We used to throw ValueError, now we throw DuplicateArchitectureError which derives from ValueError

You're absolutely correct. I overlooked that. Please ignore point 2.

[RunDevelopment]

Just 2 minor nits.

[RunDevelopment]

Thank you @akx!