pgpipe error reporting

This patch updates pgpipe() on win32 to log exactly which part of the
call fails when it does. (As it is now, there is no way to figure out
the point of error). Shouldn't be a problem since it's most defintily
not a performance-critical path (only called on pgstat startup ATM).

This should help us debug the pipe error message that's on the win32
status page (which I myself have never been able to reproduce, and thus
haven't figured out a better way to debug yet)


/Magnus

 <<pipe_errmsg.patch>>