Fix for failure with geth 1.5.0 (web3.eth.coinbase throws exception)

[d10r]

I'm running geth 1.5.0-unstable-f130c5d3.
gethdev fails with

Fatal: Failed to execute /home/didi/dev-tools/node_4.4.3/lib/node_modules/gethdev/gethdev.js: etherbase address must be explicitly specified

I could track this down to an exception being thrown by web3.eth.coinbase when no account exists:

     > web3.eth.coinbase
etherbase address must be explicitly specified
    at web3.js:3119:20                                                                                                                                                                       
    at web3.js:6023:15                                                                                                                                                                       
    at get (web3.js:5923:38)                                                                                                                                                                 
    at <anonymous>:1:-1 

Provided is a fix which works for both the old and new behaviour.

[amacneil]

Sorry for the slow response, happy to merge this if you can fix the lint issues being reported by CI.