When modeling a process with a control flow from a boundary to an error event a constraint violation is shown mentioning that the end event must be connected to a start event.
A workaround is to connect the control flow coming from the boundary event to an already existing end event that can be reached from the start event without passing a boundary event.
In the worst case, if the end event has to be used only after the boundary event, an exclusive choice gateway can be used as a workaround. In this case the exclusive choice gateway should be inserted into an uncritical path of the process and a conditional sequence connector with the condition false connected from it to the end event.
The previous flow should be preserved by the default flow of the exclusive choice gateway.
So at runtime the right execution path will be chosen but the constraint doesn't avoid the build of the process.