shada: Translate errors and add error codes

Notes:
- E136 code greatly changed its meaning: now it is write error and not read
  error.
- E195 was removed because shada_read_everything will already do all the
  necessary error reporting.
- E886 can be reported by both :rshada and :wshada, but :rshada comes first and
  AFAIR it is the only error which is not E575 and can be reported by :rshada.

src/nvim/main.c

@@ -383,7 +383,7 @@ int main(int argc, char **argv)
    * This is where v:oldfiles gets filled.
    */
   if (*p_shada != NUL) {
-    shada_read_everything(NULL, false);
+    shada_read_everything(NULL, false, true);
     TIME_MSG("reading ShaDa");
   }
   /* It's better to make v:oldfiles an empty list than NULL. */