Raise helpful exceptions for irradiance.gti_dirint

[cwhanse]

• Closes gti_dirint raises 'UnboundLocalError: local variable 'best_ghi' referenced before assignment' #1342
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gti_dirint could fail for a number of not-well-defined conditions. One known condition is reported in #1342, where all inputs have AOI >= 90.

I could not determine exactly how much data is "required" by gti_dirint. For instance, if there are insufficient daily data with 60 < AOI < 85 (see these lines), the function will proceed with kt_prime_gte_90= np.nan and will basically assume that the day is entirely clear at this line. I can't say that's a failure of the code, but I'm skeptical that gti_dirint will return anything meaningful. I don't know if we want to check, or what exception or warning to raise.

In any case, the check added in this PR closes the issue reported in #1342, and sets up gti_dirint to have additional checks added, should that be desired.