fix(js-cli): Provide a helpful error message if devPath is not specified

[rajivshah3]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• It's submitted to the dev branch and not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Currently, running tauri dev when a devPath is not specified in tauri.conf.js returns the following error:

(node:10828) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'startsWith' of undefined
    at Runner.run (/Users/rajiv/.config/yarn/global/node_modules/tauri/runner.js:45:37)
    at Object.<anonymous> (/Users/rajiv/.config/yarn/global/node_modules/tauri/bin/tauri-dev.js:34:7)
    at Module._compile (internal/modules/cjs/loader.js:959:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:995:10)
    at Module.load (internal/modules/cjs/loader.js:815:32)
    at Function.Module._load (internal/modules/cjs/loader.js:727:14)
    at Module.require (internal/modules/cjs/loader.js:852:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at tauri (/Users/rajiv/.config/yarn/global/node_modules/tauri/bin/tauri.js:33:5)
    at Object.<anonymous> (/Users/rajiv/.config/yarn/global/node_modules/tauri/bin/tauri.js:40:1)

Providing a message such as Error: devPath not specified, please check your tauri.conf.js will help users figure out what's wrong without having to look through the source code of the CLI to find what Cannot read property 'startsWith' of undefined means.

[nothingismagick]

Thanks @rajivshah3 - but I spoke briefly with Lucas right now - and we think it would be better to do the validation in tauri-config.js- because that separates the concerns better. But otherwise your intuition is spot on.

[rajivshah3]

we think it would be better to do the validation in tauri-config.js

Yeah, that makes more sense. This was more of a hotfix for something I ran into a couple hours ago but I also agree that the validation should be done outside the runner