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 variable. 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.