Support better chaining of errors with error_chain

Support better chaining of errors, e.g. with the error-chain crate,

https://github.com/rust-lang-nursery/error-chain

Context:

Better passing of a header/entry mismatch error custom error that I've made into a validation error added here:

holochain-rust/crates/core/src/network/entry_with_header.rs

Lines 20 to 26 in e5410ce

    
           pub fn try_from_entry_and_header( 
        
               entry: Entry, 
        
               header: ChainHeader, 
        
           ) -> Result<EntryWithHeader, HolochainError> { 
        
               if entry.address() != *header.entry_address() { 
        
                   Err(HolochainError::ValidationFailed(String::from( 
        
                       "Entry/Header mismatch",

I see that there is custom code for error handling in https://github.com/holochain/holochain-rust/blob/develop/crates/core_types/src/error/mod.rs, but unless I am mistaken, it seems like in practice errors may not be chained, e.g. as shown in the above example, where it seems like it would be better to have a custom error for the entry/header mismatch.

Perhaps #395 would address this, as noted here:

https://github.com/holochain/holochain-rust/blob/develop/crates/core_types/src/error/mod.rs#L40-L41

	pub fn try_from_entry_and_header(
	entry: Entry,
	header: ChainHeader,
	) -> Result<EntryWithHeader, HolochainError> {
	if entry.address() != *header.entry_address() {
	Err(HolochainError::ValidationFailed(String::from(
	"Entry/Header mismatch",