Replace libvlc_exception_get_message with libvlc_errmsg

(This is a bit hackish as the conversion is not completed)

Sources/VLCLibrary.m

@@ -42,7 +42,7 @@ void __catch_exception( void * e, const char * function, const char * file, int
         NSException* libvlcException = [NSException
             exceptionWithName:@"LibVLCException"
             reason:[NSString stringWithFormat:@"libvlc has thrown us an error: %s (%s:%d %s)", 
-                libvlc_exception_get_message( ex ), file, line_number, function]
+                libvlc_errmsg(), file, line_number, function]
             userInfo:nil];
         libvlc_exception_clear( ex );
         @throw libvlcException;