tdb2: actually implement tdb_error().

I'm still not sure it'll last into the final version, but finish the
implementation.

ccan/tdb2/doc/TDB1_porting.txt

@@ -4,8 +4,10 @@ Interface differences between TDB1 and TDB2.
   TDB_DATA typedef if you want portability between the two.
 
 - tdb2 functions return 0 on success, and a negative error on failure,
-  whereas tdb1 functions returned 0 on success, and -1 on failure.  tdb1
-  then used tdb_error() to determine the error.
+  whereas tdb1 functions returned 0 on success, and -1 on failure.
+  tdb1 then used tdb_error() to determine the error; this is also
+  supported in tdb2 to ease backwards compatibility, though the other
+  form is preferred.
 
 - tdb2's tdb_fetch() returns an error, tdb1's returned the data directly
   (or tdb_null, and you were supposed to check tdb_error() to find out why).

ccan/tdb2/tdb.c

@@ -382,6 +382,11 @@ const char *tdb_errorstr(enum TDB_ERROR ecode)
 	return "Invalid error code";
 }
 
+enum TDB_ERROR tdb_error(struct tdb_context *tdb)
+{
+	return tdb->last_error;
+}
+
 enum TDB_ERROR COLD tdb_logerr(struct tdb_context *tdb,
 			       enum TDB_ERROR ecode,
 			       enum tdb_log_level level,