[TASK] getConfigValue without FlexForm sheet

The getConfigValue method in tx_jkpoll_pi1 does not require a FlexForm
sheet any more. If no sheet is set the method will not try to override
the config value with a FlexForm setting.
[BUGFIX] Fix display of login error message

If no user was logged in voting was not possible he would always get an
error message, that he is not logged in or he already voted.

With this fix the message will only be displayed if the user check is
enabled.
[FEATURE] Configurable error container classes

There are now two different configurable error classes.

One is used for fatal errors during processing (error_container_class).

The other one is used for the message that tells the user that he is not
allowed to vote on the current poll (unvoteable_message_container_class).
[TASK] Translation improvements