This commit contains multiple improvements to error reporting in jsonpath
including but not limited to getting rid of following things:

 * definition of error messages in macros,
 * errdetail() when valueable information could fit to errmsg(),
 * word "singleton" which is not properly explained anywhere,
 * line breaks in error messages.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/14890.1555523005%40sss.pgh.pa.us
Author: Alexander Korotkov
Reviewed-by: Tom Lane