Parse and validate all cli arguments before launching pa11y

[josebolos]

When launching pa11y from the command line with an invalid runner as an argument, pa11y crashes instead of returning a helpful error message.

$ pa11y --runner potato https://www.nature.com

Welcome to Pa11y

 > Running Pa11y on URL https://www.nature.com
(node:4782) UnhandledPromiseRejectionWarning: Error: Cannot find module 'potato'
Require stack:
- /Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js
- /Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/bin/pa11y.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:965:15)
    at Function.Module._load (internal/modules/cjs/loader.js:841:27)
    at Module.require (internal/modules/cjs/loader.js:1025:19)
    at require (internal/modules/cjs/helpers.js:72:18)
    at loadRunnerFile (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:445:9)
    at loadRunnerScript (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:474:23)
    at injectRunners (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:330:39)
    at runPa11yTest (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:124:8)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async pa11y (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:39:18)
(node:4782) 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(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:4782) [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.
(node:4782) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)

Error: Cannot find module 'potato'
Require stack:
- /Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js
- /Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/bin/pa11y.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:965:15)
    at Function.Module._load (internal/modules/cjs/loader.js:841:27)
    at Module.require (internal/modules/cjs/loader.js:1025:19)
    at require (internal/modules/cjs/helpers.js:72:18)
    at loadRunnerFile (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:445:9)
    at loadRunnerScript (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:474:23)
    at injectRunners (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:330:39)
    at runPa11yTest (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:124:8)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async pa11y (/Users/jose.bolos/.nvm/versions/node/v12.18.3/lib/node_modules/pa11y/lib/pa11y.js:39:18)

---

Expected behaviour

Pa11y should validate all arguments and return a helpful error message if the arguments are invalid before attempting to do anything else.

Actual behaviour

Crash + stack.

Steps to reproduce

1. Install pa11y 6 alpha: npm install pa11y@next
2. Run pa11y --runner potato https://www.nature.com

Environment

  System:
    OS: macOS 10.15.4
    CPU: (8) x64 Intel(R) Core(TM) i7-8559U CPU @ 2.70GHz
    Memory: 384.65 MB / 16.00 GB
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 12.18.3 - ~/.nvm/versions/node/v12.18.3/bin/node
    npm: 6.14.6 - ~/.nvm/versions/node/v12.18.3/bin/npm