pgsql: When creating a collation, check that the locales can be loaded

When creating a collation, check that the locales can be loaded

This is the same check that would happen later when the collation is
used, but it's friendlier to check the collation already when it is
created.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/4442e1975d3c4c96a0b573b7abd864b0cbe26f9d

Modified Files
--------------
src/backend/commands/collationcmds.c             |    9 ++++++++-
src/test/regress/expected/collate.linux.utf8.out |    2 ++
src/test/regress/sql/collate.linux.utf8.sql      |    1 +
3 files changed, 11 insertions(+), 1 deletions(-)