Improve error handling for the waitForXX() methods on Windows

The slots _q_completeXX() returns empty values, therefore the
waitForXX() methods continue to work even if occurs an error
from the I/O completion.

Is reasonable allow to return of boolean values for the I/O
completion slots.

Besides, it allows avoiding indirect detection of an error,
comparing of the sizes of I/O buffers before and after the
operation completion. Though, for the waitForReadyRead(),
for this purpose it is necessary to add additional
modifications in a following patches.

Tested on Windows 7/8 with the virtual com0com ports using
Qt4 and then Qt5. Testing was made using of autotests
and examples.

Change-Id: I95a76461af4595f6658f0cad766a4fff14eb7afc
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>