Improve error reporting for problems in text search configuration files

by installing an error context subroutine that will provide the file name
and line number for all errors detected while reading a config file.
Some of the reader routines were already doing that in an ad-hoc way for
errors detected directly in the reader, but it didn't help for problems
detected in subroutines, such as encoding violations.

Back-patch to 8.3 because 8.3 is where people will be trying to debug
configuration files.

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