[Rust][DataFusion] Improve the errors that result when a aggregate type is not supported

When you try and run a query such as

select AVG(ts_colum) from t;

where ts_column has `DataType::Timestamp` type, you get a pretty unintelligible error message

"Coercion from [Timestamp(Nanosecond, None)] to the signature Uniform(1, [Int8, Int16, Int32, Int64, UInt8, UInt16, UInt32, UInt64, Float32, Float64]) failed."

This error should be improved to say something more like "AVG is not supported for

try an explicit cast."