use a distinct error_chain for the tdb submodule

src/errors.rs

@@ -1,9 +1,5 @@
 error_chain!{
-    foreign_links {
-        Io(::std::io::Error);
-        StrFromUtf8(::std::str::Utf8Error);
-        StringFromUtf8(::std::string::FromUtf8Error);
-        StringFromUtf16(::std::string::FromUtf16Error);
+    links {
+        Tdb2Error(::tdb2::errors::Error, ::tdb2::errors::ErrorKind);
     }
-
 }