core: stdcm: when we can't find a mareco standard allowance, return a linear allowance envelope

[eckter]

During STDCM postprocessing, we try to apply the standard allowance as a mareco allowance, while avoiding conflicts. We sometimes fail to find such an allowance (because of various bugs that aren't easily fixed).

Instead of returning a 500 response, we should try to return a linear allowance as an approximation.

The changes should be contained within this file. It should affect both the assertion about duplicate range transition and the exception when reaching the end of the loop.