raise specific error when no update was possible

[brettfo]

When the NuGet updater cannot perform any update, jobs currently fail with unknown_error which comes from here:

dependabot-core/updater/lib/dependabot/dependency_change_builder.rb

Line 73 in dddd7b2

raise DependabotError, "FileUpdater failed" unless updated_files.any?

This isn't very descriptive and makes it difficult to investigate the unknown_error bucket, especially since in this case we know exactly what went wrong.

The fix is to add a new error type that represents this scenario and raise it directly in the NuGet updater. This has the benefit of making the call stack more usable.

The error type string update_not_possible is already well-known.