clarify WETBULBTMP error message

[wknoben]

summa/build/source/engine/conv_funcs.f90

Line 344 in 19a7f9c

if(iter==maxiter)stop 'failed to converge in WETBULBTMP'

A typical cause for this is either missing forcing data or incorrect units. Would be helpful to update this print to show exactly which input values it failed on for quicker diagnosis.