ethrpc: preserve non-batch errors in BatchCall

[pengqima]

Preserve the original error returned by Provider.Do when BatchCall receives an error that is not a BatchError.

The previous type assertion reused the name err:

if err, ok := err.(BatchError); ok {

When the assertion failed, the else branch referred to the zero value produced by the failed assertion instead of the original error. This resulted in an empty error such as:

unable to batch call:

It also prevented callers from inspecting the underlying error with errors.Is.

The asserted value is now named batchErr, allowing the non-batch branch to wrap the original error. Existing BatchError handling remains unchanged.

A regression test uses a failing HTTP client to verify that the transport error remains in the returned error chain.