get_beacon_randomness returns OutOfGas with epoch from the future

[anorth]

When testing the new randomness syscall (implemented in built-in actors), which allows the caller to specify the epoch, @ZenGround0 received an out-of-gas when using an epoch from the future. The call should fail, but return an error corresponding to illegal argument (or possibly limit exceeded).

server returned an error response: error code 1: message execution failed: exit SysErrOutOfGas(7), revert reason: none, vm error: message failed with backtrace:
00: f01009 (method 3844450837) -- out of gas (7)
 (RetCode=7)

Called via https://github.com/filecoin-project/builtin-actors/blob/12d9af8a00d0909598c67e1a18dc1577e0833137/runtime/src/runtime/fvm.rs#L268-L280

[Stebalien]

Ah... I found the issue. The FVM is trying to return an illegal argument error in this case:

ref-fvm/fvm/src/kernel/default.rs

Lines 808 to 809 in a0f85fa

	.checked_sub(rand_epoch)
	.ok_or_else(|| syscall_error!(IllegalArgument; "randomness epoch {} is in the future", rand_epoch))?;

But ChainEpoch is an i64, not a u64. So lookback is allowed to be negative which gives us an infinite gas charge.