openssl ocsp: make index.txt parsing error more verbose

openssl ocsp: make index.txt parsing error more verbose

If index.txt exists but has some problems (like for example consisting of a single \n character in it,
or some field-number error in one of the lines) openssl will just exit without any error message.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15360)