Gracefully handle toml parse errors

[armaniferrante]

@pro-wh ran into the following when trying to compile a contract

Compiling ./contracts/Migrations.sol...
Writing artifacts to ./build/contracts

Compiling /project/oasis-box/contracts/proofs...
(node:1727) UnhandledPromiseRejectionWarning: error: failed to parse manifest at `/project/oasis-box/contracts/proofs/Cargo.toml`

Caused by:
  virtual manifests must be configured with [workspace]

(node:1727) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 4)
(node:1727) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

We should give a more informative error message.

Cargo.toml triggering this:

[dependencies]
owasm-ethereum = "0.7"
owasm-abi = "0.1"
owasm-abi-derive = "0.1"

[armaniferrante]

Solution:

Add a package section, for example,

[package]
name = "wasm-counter"
version = "0.1.0"
authors = ["Oasis Labs Inc. <info@oasislabs.com>"]

[pro-wh]

the tutorial is updated so that it uses a command to generate a skeleton Cargo.toml that already includes a working [package] section