BoundContract Call no longer returning correct solidity error

[trinhdn2]

About the behavior of the Call method in accounts/abi/bind/base.go. The correct contract error from CallContract is no longer returned, and Unpack is called even though CallContract has failed, which I think is a mistake.

And some third-party apps using abigen are relying on the old behavior, so would be great to have the correct contract error message back.

Expected behaviour

If the contract invocation has failed (due to require or else), return the correct associated error message

Actual behaviour

If the contract invocation has failed (due to require or else) "abi: attempting to unmarshall an empty string while arguments are expected" is returned