Allow sub-contexts to have different Error types with functions to inject errors into Self::Error

[soareschen]

Summary

Currently, context traits containing sub-contexts, such as HasRuntime and HasRelayTypes, require the sub-contexts such as Self::Runtime, Self::SrcChain, and Self::DstChain to have the same Error associated type as Self::Error. This was done initially for easier understanding of the code.

However, it can be restrictive to require all contexts to share the same concrete error type, especially if the concrete contexts are defined in different crates. As an example, to workaround having a common Error type, the concrete runtime context TokioRuntimeContext is parameterized with a generic Error type, so that it can set its Error associated type to be the same as the Error type in the concrete chain or relay contexts.

A better design would be to allow each contexts to have their own abstract Error types, and require injection functions to inject errors into the parent context. For example, the HasRuntime trait can be redefined as follows:

pub trait HasRuntime: InjectError<<Self::Runtime as HasError>::Error> {
    type Runtime: HasError;

    fn runtime(&self) -> &Self::Runtime;
}

When the parent context calls any methods from the sub-context, they can then use Self::inject_error to inject error types like Self::Runtime::Error into Self::Error.

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