Failed migration prints confusing message about git HEAD

[filipesilva]

This issue impacts 9.0.0-rc.0 and happened while updating https://github.com/johannesjo/super-productivity.

A failed migration can show the following message:

× Package install failed, see above.
The update failed. See above.
git HEAD was at 85f08a4d93e684566f751e38e2a2feffe0194cd9 before migrations.
×  Migration failed. See above for further details.

This message is confusing because it is not clear what to do with the message provided. This message enabled users that are proficient with git to go back to a previous commit. Less proficient users will not be able to interpret the purpose of the message.

Additionally, the message seems to indicate the last commit before the migrations. But this is incorrect, as this commit was that of the immediately previous migration:

* 558e5436 (HEAD -> update-9) update steps
* ec5e3201 migrate workspace for @angular/core@9.0.0-beta
* 85f08a4d migrate workspace for @angular/cli@9.0.0-beta.0
* 73e3ee1d Angular CLI update
* a06b9c06 update steps (before migrations)

To go back to a point before all the migrations a user would need to revert back to a06b9c06 instead.

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