Allow exception raising when connecting to IMAP

By setting `SoIMAP4ExceptionsEnabled` config key to YES

Enabled for OpenChange by default, it will ensure no action is taken
when IMAP connection is not valid.

SoObjects/Mailer/SOGoMailBaseObject.h

@@ -46,10 +46,13 @@
 {
   NSURL             *imap4URL;
   NGImap4Connection *imap4;
+  BOOL              imap4ExceptionsEnabled;
 }
 
 - (BOOL) isFolderish;
 
+- (id) init;
+
 - (id) initWithImap4URL: (NSURL *) _url
 	    inContainer: (id) _container;