Throw `Exception`s, when `PubspecBuilder` is unable to use `git`

[SleepySquash]

Synopsis

PubspecBuilder prints a warning, when it's unable to use git. However, this caused Linux build to silently fail to populate the ref value used for UpgradeWorker to determine whether application's upgrade is available or not.

Solution

This PR refactors PubspecBuilder to throw Exceptions instead of printing a warnings, when git isn't available.

The only downside is longer execution of checkout step: it'll grow from 1-2 seconds up to 5-10 seconds for each job, because GitHub Runner will have to fetch the whole Git history instead of only the branch it's on.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains issue reference
  • [x] Has type and k:: labels applied
• Before review:
  • [x] Documentation is updated (if required)
  • [x] Tests are updated (if required)
  • [x] Changes conform code style
  • [x] CHANGELOG entry is added (if required)
  • [x] FCM (final commit message) is posted or updated
  • [x] Draft mode is removed
• [x] Review is completed and changes are approved
  • [x] FCM (final commit message) is approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] All temporary labels are removed

[SleepySquash]

FCM

Refactor `PubspecBuilder` to throw `Exception` when Git can't be executed (#1054)