The Greenplum specific error handling via ereport()/elog() calls was
in need of a unification effort as some parts of the code was using a
different messaging style to others (and to upstream). This aims at
bringing many of the GPDB error calls in line with the upstream error
message writing guidelines and thus make the user experience of
Greenplum more consistent.

The main contributions of this patch are:

* errmsg() messages shall start with a lowercase letter, and not end
  with a period. errhint() and errdetail() shall be complete sentences
  starting with capital letter and ending with a period. This attempts
  to fix this on as many ereport() calls as possible, with too detailed
  errmsg() content broken up into details and hints where possible.

* Reindent ereport() calls to be more consistent with the common style
  used in upstream and most parts of Greenplum:

	ereport(ERROR,
			(errcode(<CODE>),
			 errmsg("short message describing error"),
			 errhint("Longer message as a complete sentence.")));

* Avoid breaking messages due to long lines since it makes grepping
  for error messages harder when debugging. This is also the de facto
  standard in upstream code.

* Convert a few internal error ereport() calls to elog(). There are
  no doubt more that can be converted, but the low hanging fruit has
  been dealt with. Also convert a few elog() calls which are user
  facing to ereport().

* Update the testfiles to match the new messages.

Spelling and wording is mostly left for a follow-up commit, as this was
getting big enough as it was. The most obvious cases have been handled
but there is work left to be done here.

Discussion: https://github.com/greenplum-db/gpdb/pull/6378Reviewed-by: NAshwin Agrawal <aagrawal@pivotal.io>
Reviewed-by: NHeikki Linnakangas <hlinnakangas@pivotal.io>