primitives: replace error propagation with `expect`

This error path cannot happen, and if it could, failing formatting is
not the correct source of action, because the std docs say that
formatting may only fail if the formatter says so.

Fixes comment in #4269

primitives/src/block.rs

@@ -223,7 +223,7 @@ impl fmt::Display for Header {
             self.time.to_u32().to_le_bytes().as_hex(),
             self.bits.to_consensus().to_le_bytes().as_hex(),
             self.nonce.to_le_bytes().as_hex(),
-        )?;
+        ).expect("total length of written objects is 160 characters");
         fmt::Display::fmt(&buf, f)
     }
 }