pgsql: Improve log messages related to tablespace_map file

Improve log messages related to tablespace_map file

This patch changes the log message which is logged when the server
successfully renames backup_label file to *.old but fails to rename
tablespace_map file during the shutdown. Previously the WARNING
message "online backup mode was not canceled" was logged in that case.
However this message is confusing because the backup mode is treated
as canceled whenever backup_label is successfully renamed. So this
commit makes the server log the message "online backup mode canceled"
in that case.

Also this commit changes errdetail messages so that they follow the
error message style guide.

Back-patch to 9.5 where tablespace_map file is introduced.

Original patch by Amit Kapila, heavily modified by me.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/fb98859ea0cac3f85041ca052bfdd0b1c8814932

Modified Files
--------------
src/backend/access/transam/xlog.c | 36 ++++++++++++++++++++----------------
1 file changed, 20 insertions(+), 16 deletions(-)