Handle exceptions raised by eth_estimateGas

[iamdefinitelyahuman]

Related issue: trufflesuite/ganache#462
Related PR: trufflesuite/ganache#457

Changes in ganache-core have broken brownie when estimating gas for a would-be-failing transaction. This new behavior is consistent with geth or parity, however, so this issue was always present when using brownie for a non-local environment.

We should not assume that all users will be on the most up-to-date version of Ganache, or that the behavior won't change again or be handled differently by some other RPC client in the future - the call should be attempted, and the exception handled properly if it fails. Since Brownie requires
broadcasting failed tx's as part of running tests and evaluating coverage, the solution here might be to set the gas limit as == the block gas limit when an exception is raised.