Change RPC error type from io::Error => RpcError.

Becaue tarpc is a library, not an application, it should strive to
use structured errors in its API so that users have maximal flexibility
in how they handle errors. io::Error makes that hard, because it is a
kitchen-sink error type.

RPCs in particular only have 3 classes of errors:

- The connection breaks.
- The request expires.
- The server decides not to process the request.

(Of course, RPCs themselves can have application-specific errors, but
from the perspective of the RPC library, those can be classified as
successful responsees).

plugins/src/lib.rs

@@ -746,7 +746,7 @@ impl<'a> ServiceGenerator<'a> {
                     #[allow(unused)]
                     #( #method_attrs )*
                     #vis fn #method_idents(&self, ctx: tarpc::context::Context, #( #args ),*)
-                        -> impl std::future::Future<Output = std::io::Result<#return_types>> + '_ {
+                        -> impl std::future::Future<Output = Result<#return_types, tarpc::client::RpcError>> + '_ {
                         let request = #request_ident::#camel_case_idents { #( #arg_pats ),* };
                         let resp = self.0.call(ctx, #request_names, request);
                         async move {