`heroku ci:run` needs explanatory warnings if the current git repo is in detached head state

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Do you want to request a *feature* or report a *bug*?
-----------------------------------------------------
Feature, I guess. Though it depends on whether one considers error handling/logging to be a feature or the lack thereof to be a bug.

What is the current behavior?
-----------------------------
When running `heroku ci:run` from a detached head state, the Heroku CLI fails silently, leaving no explanation or warnings about the reasoning behind this behavior.

Suppose we're working on a git repo `my-webserver`, configured to deploy to the Heroku app `my-heroku-webserver`.

The following sequence of commands works fine:
```
cd my-webserver
git checkout dev
heroku ci:run --app my-heroku-webserver
```
The tests will build and run as configured, yay!

However, if we instead checkout to a specific commit and thus enter a detached head state:
```
cd my-webserver
git checkout 0a2dac
heroku ci:run --app my-heroku-webserver
```
Then the Heroku CLI exits silently with an exit code of 1. The error code 1 is not the issue here (see below) -- the issue is that there is no warning/error message or logging explaining the failure.

What is the expected behavior?
------------------------------
To be clear, I am NOT proposing that `heroku ci:run` should work in a detached head state -- we need to know the branch name, so it stands to reason that we must be on a branch. (For the details of what's exactly going on, we get the branch name [here](https://github.com/heroku/cli/blob/master/packages/ci/src/commands/ci/run.ts#L26) by [calling `git --symbolic-ref --short HEAD`](https://github.com/heroku/cli/blob/master/packages/ci/src/utils/git.ts#L73).)

However, it would probably be a good idea to either pass through the `stderror` from the `git` call to the user on failure or else to explain why the command is failing.

Until that's set up, I figured it might be good to get an issue up for this -- I ran into this while trying to run the Heroku CLI inside GitHub actions, which by default checks out a SHA, not a branch. Hopefully this can help others who are banging their heads against the same issue until the quick fix for this eventually works its way in.