Proper error handling in EtherRouter

[elenadimitrova]

Since delegatecalls return a value indicates success or failure of the call, we use an assert in EtherRouter to check that. Since the introduction or the different validators: assert, require and revert however calls that fail the gas-efficient revert in contracts still consume all gas because of the EtherRouter assert check.
Here we've switched that assert to a require which eliminates this problem returning the unspent gas in failed requires and consuming all the gas still in failed asserts down the call chain from EtherRouter.

When using EtherRouter and further down the chain an assert fails, not all sent gas is spent in the overall transaction, but ~73000 gas less than the total.

Further we improve and simplify test checks for errors in two: checkErrorAssert and checkErrorRevert where both check

• the transaction receipt return value is 0 (for failure)
• if client is testrpc we ensure the return error message is a revert (from EtherRouter)
• also we validate gas usage for checkErrorAssert to differentiate from checkErrorRevert

Note that an upgrade of the parity client was required due to a fix there for openethereum/parity-ethereum#6920
This takes us to Beta version 1.8.3

This PR also enables the test suite to be run under parity by adding a check in testHelperer.forwardTime function that skips any tests that calls it, since this is not yet implemented in parity