pgsql: Improve error reporting when location specified by postgres -D d

Improve error reporting when location specified by postgres -D does not exist

Previously, the first error seen would be that postgresql.conf does not
exist.  But for the case where the whole directory does not exist, give
an error message about that, together with a hint for how to create one.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ac7238dc0faccb0ad077aa9922df6e75b0b1bda3

Modified Files
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src/backend/utils/misc/guc.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)