Report GetLastError value when DebugActiveProcess fails

Report GetLastError value when DebugActiveProcess fails

When DebugActiveProcess fails, the error message is fairly generic:

    error (_("Can't attach to process."));

It would be more useful for diagnosing problems if the Windows error
code was included in the message.  This patch implements this.

gdb/ChangeLog
2019-11-19  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_nat_target::attach): Include GetLastError
	result in error when DebugActiveProcess fails.

Change-Id: Ie1bf502a0d96bb7c09bd5b1c5e0c924ba58cd68c