fix: add better error messages for a couple of error cases

[larus]

Purpose of this PR

add better error messages for a couple of error cases

• When using a generic INetworkSerializable+IEquatable combo and update documentation with workaround
• When there are no initialized elements in the network prefab list (fix null reference exception)

Example error printed with the generic IEquatable error is

Unity.Netcode.Editor.CodeGen.NetworkBehaviourILPP: (0,0): error  - TestClassNotWorking`1<System.Int32>: A generic IEquatable 'System.IEquatable`1<TestClassNotWorking`1<T>>' is not supported.

which is printed in addition to the usual error for not implementing IEquatable.

Jira ticket

MTTB-1202
MTTB-1306

Changelog

• Fixed: Exception when the network prefab list in the network manager has uninitialized elements.
• Changed: Better error message when using generic IEquatable in a generic INetworkSerializable class and updated documentation with workaround.

Documentation

• Includes edits to existing public API documentation.

Testing & QA (How your changes can be verified during release Playtest)

Manually tested the codegen errors with IEquatable.

Functional Testing

Manual testing :

• Manual testing done

Automated tests:

• Covered by existing automated tests
• Covered by new automated tests

Does the change require QA team to:

• Review automated tests?
• Execute manual tests?
• Provide feedback about the PR?

If any boxes above are checked the QA team will be automatically added as a PR reviewer.

Backports

coming if needed

[NoelStephensUnity]

Just made some minor tweaks, but the rest looks good!
👍
Thank you Laurus! 🙌

[EmandM]

Thanks for the contribution! Just some small notes.

I think there might also be an issue when the list itself has a null prefab. I'm not sure if this fix catches that?

[larus]

I think there might also be an issue when the list itself has a null prefab. I'm not sure if this fix catches that?

This is already handled, this will emit an error message