Unclear error message when testrpc isn't running

[ripper234]

Issue

The error message when running truffle test without running testrpc is confusing.

Steps to Reproduce

Run truffle test without running testrpc.

Expected Behavior

A beginner-friendly error message that explains you should run testrpc before running truffle test

Actual Results

truffle test
Error: Invalid JSON RPC response: ""
    at Object.InvalidResponse (/mnt/c/Users/User/AppData/Roaming/nvm/v7.5.0/node_modules/truffle/node_modules/web3/lib/web3/errors.js:35:16)
    at XMLHttpRequest.request.onreadystatechange (/mnt/c/Users/User/AppData/Roaming/nvm/v7.5.0/node_modules/truffle/node_modules/web3/lib/web3/httpprovider.js:115:32)
    at XMLHttpRequestEventTarget.dispatchEvent (/mnt/c/Users/User/AppData/Roaming/nvm/v7.5.0/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:64:18)
    at XMLHttpRequest._setReadyState (/mnt/c/Users/User/AppData/Roaming/nvm/v7.5.0/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:354:12)
    at XMLHttpRequest._onHttpRequestError (/mnt/c/Users/User/AppData/Roaming/nvm/v7.5.0/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:544:12)
    at ClientRequest.<anonymous> (/mnt/c/Users/User/AppData/Roaming/nvm/v7.5.0/node_modules/truffle/node_modules/xhr2/lib/xhr2.js:414:24)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:191:7)
    at Socket.socketErrorListener (_http_client.js:358:9)
    at emitOne (events.js:96:13)

Environment

• Operating System: Windows
• Truffle version: 3.2.1

[tcoulter]

Thanks! We'll be making this error message better. See #489.