tests/qml: fix various issues with the MessageOptions test

This commit fixes three issues related to tst_MessageOptions.qml:
1) There was no test to check that the call to createSIPAccount() at the
   beginning of test_createMessageOptionsPopup() actually succeeded
2) The created account was not deleted at the end of the test
3) The test would occasionally crash due to an uncaught
   std::out_of_range exception thrown by the
   AccountModelPimpl::slotAccountDetailsChanged function. I've modified
   the function to issue a warning instead, which makes it consistent
   with AccountModelPimpl::slotVolatileAccountDetailsChanged and avoids
   throwing an exception from a Qt slot (which is considered undefined
   behavior according to the Qt documentation)

Change-Id: I4d939b29f27ee27de673aea7046997ef756fbcee