EoS workchain: return results also when calculation at some volumes fail

[bosonie]

In the current implementation of the EoS workchain, if any of the calculated volumes fails, the workchain finishes with error "ERROR_SUB_PROCESS_FAILED" and no output is returned in the workchain. This is inconvenient since it might happen that few out of many calculated volumes fail and not having outputs makes more difficult to retrieve info on the calculations completed successfully. On the other hand, it is also incorrect to return exit status zero when one calculation did not finish. An option could be to return the energy and volume of all the successful calculations AND return an error if any of the calculations failed. Another option could be to define a rate of success that is acceptable, and take action based on that.