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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[gitguardian]

⚠️ GitGuardian has uncovered 2 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
-	-	GitHub App Keys	ccbbfd8	database/seeders/GithubAppSeeder.php	View secret
-	-	Generic Password	e1bcae7	templates/compose/resend.yaml	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}