patch 8.2.3967: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.
diff --git a/src/mbyte.c b/src/mbyte.c
index bbbb506..e9febcb 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4907,7 +4907,7 @@
 	if (verbose && p_verbose > 0)
 	{
 	    verbose_enter();
-	    semsg(_(e_loadlib),
+	    semsg(_(e_could_not_load_library_str_str),
 		    hIconvDLL == 0 ? DYNAMIC_ICONV_DLL : DYNAMIC_MSVCRT_DLL,
 		    GetWin32Error());
 	    verbose_leave();
@@ -4930,7 +4930,7 @@
 	if (verbose && p_verbose > 0)
 	{
 	    verbose_enter();
-	    semsg(_(e_loadfunc), "for libiconv");
+	    semsg(_(e_could_not_load_library_function_str), "for libiconv");
 	    verbose_leave();
 	}
 	return FALSE;
@@ -5524,7 +5524,7 @@
 
     if (argvars[0].v_type != VAR_LIST || argvars[0].vval.v_list == NULL)
     {
-	emsg(_(e_listreq));
+	emsg(_(e_list_required));
 	return;
     }
     l = argvars[0].vval.v_list;