Improved error messages for `estimateGas` failures

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":""}
```