Log error when quasar config prep fails

[ovx]

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

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

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:

• 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)
• It's been tested on Windows
• It's been tested on Linux
• It's been tested on MacOS
• Any necessary documentation has been added or updated in the docs (for faster update click on "Suggest an edit on GitHub" at bottom of page) or explained in the PR's description.

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:
When a quasar.conf.js contains JS errors, the build tool logs a vague message '[FAIL] quasar.conf.js has JS errors' without any specific details. This change logs out the error stack after the fail message.

[rstoenescu]

Thanks!

[rstoenescu]

Available in CLI 0.15.15