[Live mode] Fix buttons getting out of sync on error.

By propagating the error that occurs when we call
core.startContinuousSequenceAcquisition() up the call stack, we can ensure
that the Start/Stop Live button doesn't get out of sync with the state of
the program.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14639 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd