pgsql: Improve error message and hint for ALTER COLUMN TYPE can't-cast

Improve error message and hint for ALTER COLUMN TYPE can't-cast failure.

We already tried to improve this once, but the "improved" text was rather
off-target if you had provided a USING clause. Also, it seems helpful
to provide the exact text of a suggested USING clause, so users can just
copy-and-paste it when needed. Per complaint from Keith Rarick and a
suggestion from Merlin Moncure.

Back-patch to 9.2 where the current wording was adopted.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b00982344a73d9cb626430dd17a6da84c15c9980

Modified Files
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src/backend/commands/tablecmds.c | 25 ++++++++++++++++++++-----
src/test/regress/expected/alter_table.out | 7 +++++--
src/test/regress/sql/alter_table.sql | 1 +
3 files changed, 26 insertions(+), 7 deletions(-)