pgsql: Improve error message for ALTER COLUMN TYPE coercion failure.

Improve error message for ALTER COLUMN TYPE coercion failure.

Per recent discussion, the error message for this was actually a trifle
inaccurate, since it said "cannot be cast" which might be incorrect.
Adjust that wording, and add a HINT suggesting that a USING clause might
be needed.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/488c6dd1708a2c4a9e9f307001f28c05c48651bd

Modified Files
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src/backend/commands/tablecmds.c | 7 ++++---
src/test/regress/expected/alter_table.out | 8 +++++---
2 files changed, 9 insertions(+), 6 deletions(-)