Update messages in preparation for i18n

Many messages will be marked for translation in the following
commits. This commit updates some of them to be more consistent and
reduce diff noise in those commits. Changes are

- keep the first letter of die(), error() and warning() in lowercase
- no full stop in die(), error() or warning() if it's single sentence
  messages
- indentation
- some messages are turned to BUG(), or prefixed with "BUG:" and will
  not be marked for i18n
- some messages are improved to give more information
- some messages are broken down by sentence to be i18n friendly
  (on the same token, combine multiple warning() into one big string)
- the trailing \n is converted to printf_ln if possible, or deleted
  if not redundant
- errno_errno() is used instead of explicit strerror()

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -693,7 +693,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 			exit(128);
 		if (write_locked_index(&the_index, &lock,
 				       COMMIT_LOCK | SKIP_IF_UNCHANGED))
-			die (_("unable to write %s"), get_index_file());
+			die(_("unable to write %s"), get_index_file());
 		return clean ? 0 : 1;
 	} else {
 		return try_merge_command(strategy, xopts_nr, xopts,