Make some xlogreader messages more accurate

Here is a small patch to make some invalid-record error messages in
xlogreader a bit more accurate (IMO).

My starting point was that when you have some invalid WAL, you often get
a message like "wanted 24, got 0". This is a bit incorrect, since it
really wanted *at least* 24, not exactly 24. So I have updated the
messages to that effect, and also added that detail to one message where
it was available but not printed.

Going through the remaining report_invalid_record() calls I then
adjusted the use of "invalid" vs. "incorrect" in one case. The message
"record with invalid length" makes it sound like the length was
something like -5, but really we know what the length should be and what
we got wasn't it, so "incorrect" sounded better and is also used in
other error messages in that file.