Replace wxLogWarning()s in wxOSX wxComboBox code with asserts.

Replace wxLogWarning()s in wxOSX wxComboBox code with asserts.

This incidentally fixes compilation when not using PCH as wx/log.h was not
included but is also more correct as the messages meant for the programmer,
and not the user, should be given using asserts and not wxLog.

Also correct some asserts messages. In particular, don't mention the function
name in the message itself as it's already shown by the assert and using it in
the string itself isn't useful and simply introduces the risk of forgetting to
update it after refactoring, as happened here in r63105.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63243 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775