Fix wxSound::Create() and IsOk() return values in wxOSX.

Don't pretend that we created wxSound object successfully without actually
doing it: this means that now passing an invalid (e.g. non-existent or using
wrong format) file to wxSound::Create()/ctor will return false/result in
IsOk() returning false later, just as in the other ports.

It also means that playing a successfully created wxSound object won't give
any error messages, as unexpectedly happened before.

(this is a backport of a6a3ad0d66 from master)

docs/changes.txt

@@ -637,6 +637,7 @@ wxOSX:
 - Fix using wxHTTP and wxFTP from worker thread.
 - Fix wxFileDialog::GetFilterIndex() for file open dialogs (phsilva).
 - Fix custom paper support (tijsv).
+- Return false from wxSound::Create()/IsOk() if the file doesn't exist.
 
 
 3.0.2: (released 2014-10-06)