console: fix error message in faux JSON-RPC responses

[fjl]

The message was used as both key and value in the error object.
This only affected unusual errors with no error code.

A common case where this occurred is connection errors:

go-ethereum >> ./build/bin/geth attach http://127.0.0.1:8545
# ... server shuts down ...
> eth.getBlock("latest")
Error: Invalid JSON RPC response: {"error":{"Post http://127.0.0.1:8545: dial tcp 127.0.0.1:8545: getsockopt: connection refused":"Post http://127.0.0.1:8545: dial tcp 127.0.0.1:8545: getsockopt: connection refused","code":-32603},"id":6,"version":"2.0"}
    at web3.js:3119:20
    at web3.js:6023:15
    at web3.js:4995:36
    at <anonymous>:1:1

The error is prettier with this PR applied:

> eth.getBlock("latest")
Error: Post http://127.0.0.1:8545: dial tcp 127.0.0.1:8545: getsockopt: connection refused
    at web3.js:3119:20
    at web3.js:6023:15
    at web3.js:4995:36
    at <anonymous>:1:1

Fixes #3528

[mention-bot]

@fjl, thanks for your PR! By analyzing the history of the files in this pull request, we identified @karalabe, @bas-vk and @cdetrio to be potential reviewers.

[karalabe]

LGTM but I'm not confident enough about how errors are returned on the RPC layer to an attached client. Could you take a look to make sure this doesn't accidentally break some error API @bas-vk ?

[karalabe]

LGTM 👍