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>

sequencer.c

@@ -306,7 +306,7 @@ static const char *action_name(const struct replay_opts *opts)
 	case REPLAY_INTERACTIVE_REBASE:
 		return N_("rebase -i");
 	}
-	die(_("Unknown action: %d"), opts->action);
+	die(_("unknown action: %d"), opts->action);
 }
 
 struct commit_message {
@@ -1444,7 +1444,7 @@ static const char *command_to_string(const enum todo_command command)
 {
 	if (command < TODO_COMMENT)
 		return todo_command_info[command].str;
-	die("Unknown command: %d", command);
+	die("unknown command: %d", command);
 }
 
 static char command_to_char(const enum todo_command command)
@@ -2608,7 +2608,7 @@ static int error_with_patch(struct commit *commit,
 			"\n"
 			"  git rebase --continue\n", gpg_sign_opt_quoted(opts));
 	} else if (exit_code)
-		fprintf(stderr, "Could not apply %s... %.*s\n",
+		fprintf_ln(stderr, "Could not apply %s... %.*s",
 			short_commit_name(commit), subject_len, subject);
 
 	return exit_code;
@@ -2719,7 +2719,7 @@ static int do_label(const char *name, int len)
 	struct object_id head_oid;
 
 	if (len == 1 && *name == '#')
-		return error("Illegal label name: '%.*s'", len, name);
+		return error("illegal label name: '%.*s'", len, name);
 
 	strbuf_addf(&ref_name, "refs/rewritten/%.*s", len, name);
 	strbuf_addf(&msg, "rebase -i (label) '%.*s'", len, name);