Several improvements for estimates, budget and costs #443

[m-mohr]

Several improvements around estimates, costs and budgets:

• Don't require that the estimated costs are the upper limit. Services may specify the costs more freely depending on their terms of service.
• If a batch job can't be estimated reliably, a EstimateComplexity error should be returned. Fixes Job estimates for UDFs etc #443
• GET /me: Clarify the behavior of the field budget, which was incorrectly inherited from the resource budget field and thus the description did not make a lot of sense.
• Minimum value for resource costs and budget is 0.