Disallow IDbFunction.HasTranslation when mapping the function to a TVF

[smitpatel]

Since TVFs are UDFs (does not map to built-in methods), having to customize translation becomes less useful. (Also parameters would be in same order too). We will throw error in model validation if IDbFunction is IsQueryable, then configuring custom translation will throw exception.

Currently it assumes that a scalar SQL will come out. If we are planning to use same IDbFunction for queryable functions too then it needs rework or breaking change.

[smitpatel]

@AndriySvyryd @ajcvickers - Looking at API, I decided to throw exception when using API directly rather than deferring to model validator. There is no step which can correct invalid configuration once API is called. Explicit throws, other configuration sources will not set custom translation and return null.