[msbuild] Show a warning if asked to load an app manifest that doesn't exist.

[rolfbjarne]

This makes it easier to diagnose problems loading/finding app manifests.

Specifying an app manifest that doesn't exist should probably be an error, but
that's very likely to break existing projects, so just make this a warning for
now.

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

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Hash: 2a3b74522dbaf6642fe929103cf4dfd50c9cd11b [PR build]

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Big Sur (11.5) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11.5) passed.

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Ventura (13.0) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13.0) passed.

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

📚 [PR Build] Artifacts 📚

Packages generated

View packages

• xamarin.ios-16.5.0.122.pkg
• xamarin.mac-9.4.0.122.pkg

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS (no change detected)
• tvOS (no change detected)
• watchOS (no change detected)
• macOS (no change detected)

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

✅ Generator diff

Generator diff is empty

Pipeline on Agent
Hash: 2a3b74522dbaf6642fe929103cf4dfd50c9cd11b [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: simulator tests.

🎉 All 225 tests passed 🎉

Tests counts

✅ bcl: All 69 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests: All 1 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 7 tests passed. Html Report (VSDrops) Download
✅ framework: All 8 tests passed. Html Report (VSDrops) Download
✅ generator: All 2 tests passed. Html Report (VSDrops) Download
✅ interdependent_binding_projects: All 7 tests passed. Html Report (VSDrops) Download
✅ install_source: All 1 tests passed. Html Report (VSDrops) Download
✅ introspection: All 8 tests passed. Html Report (VSDrops) Download
✅ linker: All 65 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ mac_binding_project: All 1 tests passed. Html Report (VSDrops) Download
✅ mmp: All 2 tests passed. Html Report (VSDrops) Download
✅ mononative: All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch: All 25 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ mtouch: All 1 tests passed. Html Report (VSDrops) Download
✅ xammac: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 8 tests passed. Html Report (VSDrops) Download
✅ xtro: All 2 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: 2a3b74522dbaf6642fe929103cf4dfd50c9cd11b [PR build]