Add indicator for successful run of MCMC to mh_history_

Currently, at the beginning of the MCMC we use metropolis_hastings_initialization to write a new mh_historyfile. But we never confirm that this run was successful. When using the load_mh_file this leads to cryptic crashes. I would propose to add an indicator for a non-crashed run to the history file and check this indicator when load_mh_file is requested. If the indicator shows a non-successful run, we should recommend either using the mh_recover option or to delete the most recent history file to make Dynare try to access the previous, hopefully successful run.