Checks input type string to avoid confusing error

[samsonradu]

When the constructor is called with non-string values we get this confusing error:

// Tk.sol
function Tk(uint256 initialSupply, string tokenName, string tokenSymbol)

// app.js
let d = self.contract.deploy({data: "", arguments: [1000000000000000, null, "tk"]});

TypeError: Cannot read property 'length' of undefined
    at ucs2decode (/Users/samsonradu/Workspace/eth/node_modules/utf8/utf8.js:26:23)
    at Object.utf8encode [as encode] (/Users/samsonradu/Workspace/eth/node_modules/utf8/utf8.js:104:20)
    at Object.utf8ToHex (/Users/samsonradu/Workspace/eth/node_modules/web3-utils/src/utils.js:168:16)
    at SolidityTypeString.formatInputString [as _inputFormatter] (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-abi/src/formatters.js:106
:24)
    at SolidityTypeString.SolidityType.encode (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-abi/src/type.js:188:17)
    at /Users/samsonradu/Workspace/eth/node_modules/web3-eth-abi/src/index.js:255:29
    at Array.map (<anonymous>)
    at ABICoder.encodeParameters (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-abi/src/index.js:254:34)
    at /Users/samsonradu/Workspace/eth/node_modules/web3-eth-contract/src/index.js:420:24
    at Array.map (<anonymous>)
    at Object._encodeMethodABI (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-contract/src/index.js:419:12)
    at Object._processExecuteArguments (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-contract/src/index.js:735:39)
    at Object._executeMethod (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-contract/src/index.js:760:54)
    at Contract.deploy (/Users/samsonradu/Workspace/eth/lib/contract.js:37:18)
    at Object.<anonymous> (/Users/samsonradu/Workspace/eth/create.js:14:3)
    at Module._compile (module.js:641:30)
    at Object.Module._extensions..js (module.js:652:10)
    at Module.load (module.js:560:32)
    at tryModuleLoad (module.js:503:12)
    at Function.Module._load (module.js:495:3)
    at Function.Module.runMain (module.js:682:10)
    at startup (bootstrap_node.js:191:16)
    at bootstrap_node.js:613:3

Checking for a valid string will at least output:

Error: Given parameter is not a valid string: undefined
    at SolidityTypeString.formatInputString [as _inputFormatter] (/Users/samsonradu/Workspace/eth/node_modules/web3-eth-abi/src/formatters.js:107:15)
...

[coveralls]

Coverage decreased (-0.03%) to 85.682% when pulling c8734d6 on samsonradu:typecheck-string-input into 6816ce5 on ethereum:1.0.

[coveralls]

Coverage decreased (-0.03%) to 85.682% when pulling c8734d6 on samsonradu:typecheck-string-input into 6816ce5 on ethereum:1.0.

[frozeman]

Good addition. The error reporting can still be improved on a lot of places.