Reword SPI_ERROR_TRANSACTION errors in PL/pgSQL

Peter Eisentraut <peter_e@gmx.net>

Commit: c9301deb9bf86aaf9144a66026bc121a3eededee
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2018-06-26T09:38:46Z
Releases: 11.0
Reword SPI_ERROR_TRANSACTION errors in PL/pgSQL

The previous message for SPI_ERROR_TRANSACTION claimed "cannot begin/end
transactions in PL/pgSQL", but that is no longer true.  Nevertheless,
the error can still happen, so reword the messages.  The error cases in
exec_prepare_plan() could never happen, so remove them.

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