[Runtime Epoch Split] (2/n) Narrow EpochManagerAdapter error type to EpochError wherever possible.

[robin-near]

As a reminder of the overall goal, we want to split RuntimeWithEpochManagerAdapter, so that any code that needs the runtime will use an Arc<RuntimeAdapter>, and any code that uses the epoch manager will use the EpochManagerHandle.

We're doing this refactoring bottom-up, i.e. propagating RuntimeWithEpochManagerAdapter around at the top-level, but making some lower-level components use Arc<RuntimeAdapter> and/or EpochManagerHandle.

That means we need to be able to obtain an Arc<RuntimeAdapter> and an EpochManagerHandle from a RuntimeWithEpochManagerAdapter. However, this is not trivial at all:

1. KeyValueRuntime, the implementation of RuntimeWithEpochManagerAdapter for testing, does not contain an EpochManager at all, so it's not possible to extract an EpochManagerHandle from it (which is essentially an arc mutex of EpochManager). That means instead of using EpochManagerHandle, we need to use Arc<EpochManagerAdapter> in the meantime.
2. Extracting an Arc<EpochManagerAdapter> from a Arc<RuntimeWithEpochManagerAdapter> is not trivial. Even though RuntimeWithEpochManagerAdapter is a trait that extends EpochManagerAdapter, trait upcast is not allowed by Rust in general. So we need to resort to a workaround.

This PR addresses an issue arising from (1), that current code that expects an EpochManagerHandle uses functions from EpochManager which mostly returns EpochError. EpochManagerAdapter, on the other hand, returns mostly near_chain_primitive::Error, so changing to the adapter type breaks existing code. So, in this PR we are changing the error type returned from most of the EpochManagerAdapter functions to also be EpochError. This is a pretty painless transition, except in some cases we need to do a two-hop conversion from EpochError to chain Error and then to some other error (and two-hop conversions cannot be supported by .into() in general), so I've defined .into_chain_error() to help with the two-hop conversion cases.

[nikurt]

Looks good.
Most of the changes are simple replacements Error -> EpochError.
The newly added into_chain_error() makes conversion of types of errors seamless.