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/shada.h

@@ -5,11 +5,11 @@ typedef long ShadaPosition;
 
 /// Flags for shada_read_file and children
 enum {
-  kShaDaWantInfo = 1,     ///< Load non-mark information
-  kShaDaWantMarks = 2,    ///< Load local file marks and change list
-  kShaDaForceit = 4,      ///< Overwrite info already read
-  kShaDaGetOldfiles = 8,  ///< Load v:oldfiles.
-  kShaDaWantHeader = 16,  ///< Do not skip header (shada_read_next_item).
+  kShaDaWantInfo = 1,       ///< Load non-mark information
+  kShaDaWantMarks = 2,      ///< Load local file marks and change list
+  kShaDaForceit = 4,        ///< Overwrite info already read
+  kShaDaGetOldfiles = 8,    ///< Load v:oldfiles.
+  kShaDaMissingError = 16,  ///< Error out when os_open returns -ENOENT.
 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS