Wrap errors returned from `Store`

[tzdybal]

In multiple places in the code, errors returned from Store are just re-returned. Because of that they are completely non-meaningful.
For example client.go:351, can return something as generic as "key not found".

All unwrapped errors should be wrapped with meaningful error message, using fmt.Errorf("meaningful message: %w", err).

[mauriceLC92]

@tzdybal Do you mind assigning this to me please?

[tzdybal]

Sure thing!

[mauriceLC92]

@tzdybal should the wrapped errors be returned from the store package?
The example in the issue relates to the rpc client so not sure how extensive the error wrapping should be

[tzdybal]

I think it will be enough (at least for now) to just wrap in store package. Then, the error message will be something like:
failed to load block: key not found and I assume someone calling RPC method will know what method returned this error.