Make the message popped up if you try to do a capture on a Win32 machine

Make the message popped up if you try to do a capture on a Win32 machine
when wpcap.dll couldn't be loaded more detailed, in the hopes that it'll
reduce the chances that somebody will see that message and not know what
to do.  Also, mention the Wiretapped.net mirror of the WinPcap site, as
the WinPcap site is all-too-often down due to networking glitches.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@3955 f5534014-38df-0310-8fa8-9805f1628bb7