Use unwind-protect to ensure action chain is inactive afterwards

[peterpaul]

The step "When I execute the action chain" does not ensure the action chain is inactive afterwards when a non-local exit (error or throw) happens during execution of the action chain.

This pull-request should fix that.

I'm not yet familiar with writing ert-tests, if you think this should be explicitely tested, please help me construct the testcase.

[peterpaul]

I could not find out how to write a unit test for this without adjusting el-mock. I've created a pull-request there (rejeep/el-mock.el#26), to support providing a lambda for a stub.

After there is any progress on that pull-request I can decide how to continue here.