Change function name and messages of detect_error.

Change function name and messages of detect_error.

This function name and message make enough misunderstandings that
there's something wrong. Actually this is harmless until the caller
through an error message, such as "admin shutdown" message. So change
the function name and messages to "extract_message" to eliminate such
misunderstandings.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2018-November/003115.html