Closes safe-global#274: Revert on failure with safeTxGas 0 (safe-glob…

…al#283)

GnosisSafe.sol

@@ -170,6 +170,9 @@ contract GnosisSafe
             // We only substract 2500 (compared to the 3000 before) to ensure that the amount passed is still higher than safeTxGas
             success = execute(to, value, data, operation, gasPrice == 0 ? (gasleft() - 2500) : safeTxGas);
             gasUsed = gasUsed.sub(gasleft());
+            // If no safeTxGas and no gasPrice was set (e.g. both are 0), then the internal tx is required to be successful
+            // This makes it possible to use `estimateGas` without issues, as it searches for the minimum gas where the tx doesn't revert
+            require(success || safeTxGas != 0 || gasPrice != 0, "GS013");
             // We transfer the calculated tx costs to the tx.origin to avoid sending it to intermediate contracts that have made calls
             uint256 payment = 0;
             if (gasPrice > 0) {