add missing errdetail for xlogreader allocation failure error

Hi,

It seems like some of the XLogReaderAllocate failure check errors are
not having errdetail "Failed while allocating a WAL reading
processor." but just the errmsg "out of memory". The "out of memory"
message without the errdetail is too generic and let's add it for
consistency and readability of the message in the server logs.

Here's a tiny patch. Thoughts?

Regards,
Bharath Rupireddy.