[content-initializer] Provide actionable feedback when content init fails

[csweichel]

Is your feature request related to a problem? Please describe

Today content initializer failure is a frustrating experience. Not only does the workspace not start, but the error message isn't helpful either.

Describe the behaviour you'd like

We must aim to provide a message/feedback which gives more insight into what went wrong, for example:

• when git operations fail, show the output of Git,
• when prebuild operations fail highlight that that's the case.

Describe alternatives you've considered

Something will always fail - there is no alternative.