Error message for mac users on outdated macOS (less than v10.15)

[JavidPack]

In the weeks since v2024.03 has been in stable, we've noticed that macOS users below v10.15 (Catalina) can't successfully launch tModLoader. The .NET 8 release notes (https://github.com/dotnet/core/blob/main/release-notes/8.0/supported-os.md#macos) say macOS 12+ is needed, but that doesn't seem to be the case.

Supported devices for v10.15 (https://support.apple.com/en-us/118458) seem to be anything in the last 10 years, so we are fine with this new limitation:

MacBook (Early 2015 or newer)
MacBook Air (Mid 2012 or newer)
MacBook Pro (Mid 2012 or newer)
Mac mini (Late 2012 or newer)
iMac (Late 2012 or newer)
iMac Pro (2017)
Mac Pro (Late 2013 or newer)

This PR adds an error message to inform the user. This is better than the game appearing to start in Steam and then suddenly closing.

TODO: Screenshot of new error message window here. Code is not tested yet:

Sample of errors seen by pre 10.15 users:
Natives (27).log
Natives (28).log

[JavidPack]

Well, haven't seen any new reports of this recently. Code looks fine, might be ok to merge without a test.

[Solxanich]

Echoed, don't see significant value in forcing a test on this relative to the % of users affected.
code review is sufficient.