Improve savepoint error messages

Peter Eisentraut <peter_e@gmx.net>

Commit: 81148856b0310e7b0e578508ace44c0aefcbee07
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2018-03-16T17:18:06Z
Releases: 11.0
Improve savepoint error messages

Include the savepoint name in the error message and rephrase it a bit to
match common style.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>

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