fix($core): optimize error log (close: #1296)

[shigma]

Summary

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

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

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

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

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

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Old non-string request resolution

Try to resolve the request as absolute/relative/dependency path and import them in chain. Use the first result as the resolved module. If any error was encounted, respective resolution will be skipped.

What are the drawbacks?

• It's not consistent with the implementation of Node.js. Node.js will take the first existing file as the final entry no matter it has errors or not.
• Referenced in Error in local theme index.js should not be ignored and reported as "cannot resolve plugin ..." #1296. A plain error log cannot resolve plugin may mislead user with an indication that the theme or plugin is not found instead of wrong.
• Whether the error is in the theme or plugin, the same error with plugin is displayed.

This PR's implementation

Try to resolve the request as absolute/relative/dependency path in chain.

• If an entry file was found, vuepress will some the try-chain and try to load the file.
• If the entry file is successfully loaded, apply it as a plugin/theme.
• If an error was encounted, vuepress will leave a error log en error was encounted in plugin "xxx".
• If all possible files do not exist, vuepress will leave a error log cannot resolve plugin "xxx".
• If similar problem was encounted in theme entry file resolution, the error log will replace plugin with theme.

[flozero]

I like definitely this PR

[vue-bot]

Hey @shigma, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚