fix: show proper error message on invalid Git source

[matpratta]

This PR fixes an issue when deploying Git repositories with Docker Compose, where the error message would incorrectly say the docker-compose.yaml file was not found instead of saying the Git source is invalid.

It should also resolve my previous issue #2563

One potential improvement here would be to do the same check everywhere that involves a Git source, possibly even before creating the Resource itself.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: app/Models/Application.php

Function	Unhandled Issue
App\Models\Application::setGitImportSettings	ErrorException: Attempt to read property "is_git_submodules_enabled" on null /app/Models/Application.php in Illuminate\Foundation\Bootstrap\H... Event Count: 7

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