`Extension` methods should not swallow errors

## Description

The `Extension` struct methods are very liberal about swallowing errors e.g. https://github.com/cli/cli/blob/41e99b9eabe47bc8262d5ebfc65626fb29a7ed87/pkg/cmd/extension/extension.go#L161-L171

I don't really think this is advisable. It makes it hard to reason about the state of the system. Is an error to read the manifest really equivalent to an extension being unpinned? No! It's an error. I don't want my pinned extension to suddenly be upgraded. In practice this probably wouldn't happen because a failure to load the extension manifest probably has the system in some other broken state.

### Expected Outcome

Errors are correctly bubbled out of these methods, or in places where errors should be swallowed for fallback, a comment exists explaining why.