pgsql: Fix lo_import and lo_export to return useful error messages more

Fix lo_import and lo_export to return useful error messages more often.

I found that these functions tend to return -1 while leaving an empty error
message string in the PGconn, if they suffer some kind of I/O error on the
file.  The reason is that lo_close, which thinks it's executed a perfectly
fine SQL command, clears the errorMessage.  The minimum-change workaround
is to reorder operations here so that we don't fill the errorMessage until
after lo_close.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/875406af6ff961ba169db11c74828f02e96f5753

Modified Files
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src/interfaces/libpq/fe-lobj.c |   33 ++++++++++++++++++++++++---------
1 files changed, 24 insertions(+), 9 deletions(-)