Tweak FOR UPDATE/SHARE error message wording (again)

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

Commit: 5712eeb2e7cf8e4236acffd4a403e7f14a48866f
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2013-07-23T18:04:36Z
Releases: 9.3.0
Tweak FOR UPDATE/SHARE error message wording (again)

In commit 0ac5ad5134 I changed some error messages from "FOR
UPDATE/SHARE" to a rather long gobbledygook which nobody liked.  Then,
in commit cb9b66d31 I changed them again, but the alternative chosen
there was deemed suboptimal by Peter Eisentraut, who in message
1373937980.20441.8.camel@vanquo.pezone.net proposed an alternative
involving a dynamically-constructed string based on the actual locking
strength specified in the SQL command.  This patch implements that
suggestion.

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