Revert "remove deprecated description for TrieError (#85)" (#86)

This reverts commit bc8a83f.

trie-db/src/lib.rs

@@ -121,6 +121,19 @@ impl<T, E> fmt::Display for TrieError<T, E> where T: MaybeDebug, E: MaybeDebug {
 	}
 }
 
+#[cfg(feature = "std")]
+impl<T, E> Error for TrieError<T, E> where T: fmt::Debug, E: Error {
+	fn description(&self) -> &str {
+		match *self {
+			TrieError::InvalidStateRoot(_) => "Invalid state root",
+			TrieError::IncompleteDatabase(_) => "Incomplete database",
+			TrieError::ValueAtIncompleteKey(_, _) => "Value at incomplete key",
+			TrieError::DecoderError(_, ref err) => err.description(),
+			TrieError::InvalidHash(_, _) => "Encoded node contains invalid hash reference",
+		}
+	}
+}
+
 /// Trie result type.
 /// Boxed to avoid copying around extra space for the `Hasher`s `Out` on successful queries.
 pub type Result<T, H, E> = crate::rstd::result::Result<T, Box<TrieError<H, E>>>;