Re #8290. Check table still exists before generating the plot.

Throw up a warning if it's gone missing because the user deleted it
manually.
Re #8290. Don't lock up the GUI if the live algorithm takes a while.

In particular, the live algorithm might be waiting for a run to start.
This change means that GUI events continue to be processed so that the
rest of MantidPlot still functions, and you can close the GUI if you
want to.
Merge branch 'bugfix/8290_step_scan_bugs' into develop