Display troubleshooting context if update command fails

[luke-engle]

What's changing?

When a user runs the update command with an invalid token, the login fails and the user gets a cryptic, unhelpful error message: Error response from daemon: Get "https://ghcr.io/v2/": denied: denied

Similarly, when a user runs the update command with a token that doesn't have the required scopes, they get this cryptic error message: Error response from daemon: denied

This change addresses this issue by providing the user with clear error messages in these scenarios.

How's this tested?

Unit tests, and running manually which you can see below 👇

Invalid token:

➜ echo $INVALID_TOKEN | dotnet run --project src/Valet/Valet.csproj update --username luke-engle --password-stdin
You are not authorized to access Valet yet. Please ensure you've completed the following:
- Requested access to Valet and received onboarding instructions via email.
- Accepted all of the repository invites sent after being onboarded.

Invalid scopes:

➜ echo $TOKEN_WITH_INVALID_SCOPES | dotnet run --project src/Valet/Valet.csproj update --username luke-engle --password-stdin
Login Succeeded

Pulling valet-customers/valet-cli:latest from the ghcr.io docker repository...
You are not authorized to access Valet yet. Please ensure you've completed the following:
- Requested access to Valet and received onboarding instructions via email.
- Accepted all of the repository invites sent after being onboarded.
- The GitHub personal access token used above contains the 'read:packages' scope.

Closes #88

[github-actions[bot]]

Unit Test Results

37 tests  +2   37 :heavy_check_mark: +2   0s :stopwatch: ±0s   1 suites ±0     0 :zzz: ±0    1 files   ±0     0 :x: ±0 

Results for commit 28e2f194. ± Comparison against base commit ccfcad4e.

This pull request removes 3 and adds 5 tests. Note that renamed tests count towards both.

``` Valet.UnitTests.AppTests ‑ CheckForUpdates_NoUpdatesNeeded("4256ea72fd01deac3e967f6b19f907587dcd6f0a976301f1aecc73dc6f146a4a","67eed1493c461efd993be9777598a456562f4e0c6b0bddcb19d819220a06dd4b","A new version of the Valet CLI is available. Run 'gh valet update' to update.\n") Valet.UnitTests.Services.DockerServiceTests ‑ UpdateImageAsync_InvalidCredentialsProvided_ReturnsFalse Valet.UnitTests.Services.DockerServiceTests ‑ UpdateImageAsync_ValidCredentialsProvided_ReturnsTrue ``` ``` Valet.UnitTests.AppTests ‑ CheckForUpdates_NoUpdatesNeeded("4256ea72fd01deac3e967f6b19f907587dcd6f0a976301f1aecc73dc6f146a4a","67eed1493c461efd993be9777598a456562f4e0c6b0bddcb19d819220a06dd4b","A new version of the Valet CLI is available. Run 'gh valet update' to update.") Valet.UnitTests.Services.DockerServiceTests ‑ UpdateImageAsync_InvalidCredentialsProvided_ThrowsUnauthorized Valet.UnitTests.Services.DockerServiceTests ‑ UpdateImageAsync_InvalidCredentialsProvided_ThrowsUnknownError Valet.UnitTests.Services.DockerServiceTests ‑ UpdateImageAsync_InvalidScopes_ThrowsUnauthorized Valet.UnitTests.Services.DockerServiceTests ‑ UpdateImageAsync_ValidCredentialsProvided_ReturnsSuccessful ```

:recycle: This comment has been updated with latest results.