Test provider connection and fail early

[gnidan]

Ref: trufflesuite/truffle#421

Example output after change:

/Users/gnidan/tmp/truffle/truffle-config/index.js:157
          throw error;
          ^

Error: Could not connect to your RPC client. Please check your RPC configuration.
    at /Users/gnidan/tmp/truffle/truffle-provider/index.js:53:17
    at /Users/gnidan/tmp/truffle/truffle-provider/node_modules/web3/lib/web3/property.js:119:13
    at /Users/gnidan/tmp/truffle/truffle-provider/node_modules/web3/lib/web3/requestmanager.js:82:20
    at XMLHttpRequest.request.onreadystatechange (/Users/gnidan/tmp/truffle/truffle-provider/node_modules/web3/lib/web3/httpprovider.js:118:13)
    at XMLHttpRequestEventTarget.dispatchEvent (/Users/gnidan/tmp/truffle/truffle-provider/node_modules/xhr2/lib/xhr2.js:64:18)
    at XMLHttpRequest._setReadyState (/Users/gnidan/tmp/truffle/truffle-provider/node_modules/xhr2/lib/xhr2.js:354:12)
    at XMLHttpRequest._onHttpRequestError (/Users/gnidan/tmp/truffle/truffle-provider/node_modules/xhr2/lib/xhr2.js:544:12)
    at ClientRequest.<anonymous> (/Users/gnidan/tmp/truffle/truffle-provider/node_modules/xhr2/lib/xhr2.js:414:24)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:188:7)

[tcoulter]

I want to add some late (but minor) requirements for this:

• Error message should include possible failure cases. Feel free to use an error message that has multiple lines and formatting, i.e.,

  Could not connect to your Ethereum client. Please check that your Ethereum client:
  
  - is running
  - is accepting RPC connections (i.e., "--rpc" option is used in geth)
  - is accessible over the network
  

  And perhaps we can have the error message link to a FAQ or document that describes this (ignore this requirement if you can't find a useful document)

• The error itself should be wrapped in truffle-error. Note that Truffle uses truffle-error as a "flag" to print out error messages nicely instead of showing a full stack trace.

[gnidan]

Closing this in favor of overriding low-level send/sendAsync inside truffle-provider