Additional database file checks in cli/Utils.unlockDatabase

Avoids prompting the user for a password if unlocking is likely to fail
due to some problem with the database file (i.e. not found, not a file,
not readable).

Add unit tests.

tests/TestCli.h

@@ -63,6 +63,7 @@ private slots:
     void testRemove();
     void testRemoveQuiet();
     void testShow();
+    void testInvalidDbFiles();
 
 private:
     QByteArray m_dbData;