Bump all nvim-specific error codes above E5000

In order to not conflict with new error codes that Vim adds, all Neovim
error codes should be above 5000.  The three existing sub-5000 error
codes (E926, E951, and E952) are now E50003, E5004, and E5005
respectively.

E953 was removed in 6167ce6df2, so just
remove it from the help.

runtime/doc/eval.txt

@@ -5069,7 +5069,7 @@ msgpackdump({list})				   {Nvim} *msgpackdump()*
 		(dictionary with zero items is represented by 0x80 byte in
 		messagepack).
 
-		Limitations:				*E951* *E952* *E953*
+		Limitations:				*E5004* *E5005*
 		1. |Funcref|s cannot be dumped.
 		2. Containers that reference themselves cannot be dumped.
 		3. Dictionary keys are always dumped as STR strings.

runtime/doc/options.txt

@@ -6512,7 +6512,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 				     *'ttyfast'* *'tf'* *'nottyfast'* *'notf'*
 'ttyfast' 'tf'		Removed. |vim-differences| {Nvim}
 
-						*'undodir'* *'udir'* *E926*
+						*'undodir'* *'udir'* *E5003*
 'undodir' 'udir'	string	(default "$XDG_DATA_HOME/nvim/undo")
 			global
 			{only when compiled with the |+persistent_undo| feature}