pgsql: Issue ERROR if FREEZE mode can't be honored by COPY

Issue ERROR if FREEZE mode can't be honored by COPY

Previously non-honored FREEZE mode was ignored.  This also issues an
appropriate error message based on the cause of the failure, per
suggestion from Tom.  Additional regression test case added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4deb57de7dcfb66f850f16c0324731fdf3d86d09

Modified Files
--------------
doc/src/sgml/ref/copy.sgml          |   12 ++++--------
src/backend/commands/copy.c         |   32 ++++++++++++++++++++------------
src/test/regress/expected/copy2.out |   18 ++++++++----------
src/test/regress/sql/copy2.sql      |    9 ++++++++-
4 files changed, 40 insertions(+), 31 deletions(-)