patch 8.2.3190: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move error messages to errors.h and give them a clear name.
diff --git a/src/digraph.c b/src/digraph.c
index 9b1836e..b354f2d 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2074,7 +2074,7 @@
 	str = skipwhite(str);
 	if (!VIM_ISDIGIT(*str))
 	{
-	    emsg(_(e_number_exp));
+	    emsg(_(e_number_expected));
 	    return;
 	}
 	n = getdigits(&str);
@@ -2571,7 +2571,7 @@
 	buflen = STRLEN(curbuf->b_p_keymap) + STRLEN(p_enc) + 14;
 	buf = alloc(buflen);
 	if (buf == NULL)
-	    return e_outofmem;
+	    return e_out_of_memory;
 
 	// try finding "keymap/'keymap'_'encoding'.vim"  in 'runtimepath'
 	vim_snprintf((char *)buf, buflen, "keymap/%s_%s.vim",