ethexe: introduce tx pool error types

[techraed]

This is mainly useful for RPC and validation:

1. RPC will send a response with a clear error message.
2. Some validation failures signalize that this error can be returned to the caller/requester, but some of them should trigger panic. So after introducing errors, panics from tx_pool::validation should be removed and called, if necessary, on upper levels.

[grishasobol]

@techraed please more details, I cannot find any panics in tx_pool::validation