Raise exception if Powheg integration checks fail
Previously this only caused a warning to be logged.
Background: some Powheg samples were found to contain a small number of events whose weight is an outlier. These could be traced back to poor phase space integration. The new behaviour is supposed to prevent such samples from being generated in the future. Instead, the integration parameters should be adjusted until the integration checks pass.
The exception can be overridden by setting the environment variable
POWHEGCONTROL_IGNORE_INTEGRATION_CHECKS to "1".
export POWHEGCONTROL_IGNORE_INTEGRATION_CHECKS=1 in bash)
In this case, a warning will be printed as in the past. Both the
warning and the exception text inform the user about this environment
This switch is useful for Powheg developers who want to run quick test
runs with bad integration, but it should not be used for physics.