msvcprobe: Read stdout in case of vswhere error
msvcprobe: Read stdout in case of vswhere error

Older versions of vswhere do not know the "-utf8" switch and running
them will result in an error. The actual error is shown in stdout
instead of stderr though. Make sure that these errors are visible to the
user.

Change-Id: Ic526ad19bc201a62be24dd43b5094c24f2551d11
Reviewed-by: Ivan Komissarov <96f14e4d07227e958f51f7d2898113639e9498d9@gmail.com>
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@qt.io>

qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs