Improved error messages for `estimateGas` failures

[shorsher]

Currently, some estimateGas failures result in an "" being passed to firefly. This seems to be because "" is the default catch in mapError.

The log message that is printed Gas estimation failed for a non-revert reason: is much more helpful than the empty string passed back to FireFly.

Example error log:

{"@timestamp":"2023-04-25T11:35:59.627Z","level":"error","message":"Gas estimation failed for a non-revert reason:  (call result: \u003cnil\u003e)","prefix":"evmconnect"}

Error reported by FireFly in failed operation:

FF10111: Error from ethereum connector: {"error":""}