Fix failing WritingSystemSetupPMTests unit tests

These failures were introduced by a bugfix yesterday.
Merge pull request #148 from sillsdev/FixFailingWritingSystemSetupPMTests

Fix failing WritingSystemSetupPMTests unit tests
Fix a failing test for a wrong exception

The fix mimics the code in the base class method.
Fix some null exception in keyboard setup

Also handle random DBus exception thrown by Linux Mint 17/Cinnamon.
This fixes https://jira.sil.org/browse/WS-284.
Merge pull request #149 from sillsdev/FixAnotherWritingSystemTestFailure

Fix a failing test for a wrong exception
Merge pull request #150 from sillsdev/Fix-WS-284

Fix some null exceptions in keyboard setup