Improve diagnostics when the correct simulators aren't available.

[rolfbjarne]

Many parts of the build use command-line tools provided by Apple, and some of these command-line tools require the correct simulator to be installed (even for device build).

If the correct simulator isn't installed, the build will fail in unhelpful ways:

error : actool exited with code 255
error : Failed to load actool log file `.../actool/asset-manifest.plist`: Failed to parse PList data type: 

We should improve this error reporting.

This is the most common way the problem manifests, but there are others as well:

• Build hangs: [CI] Intermittent actool exit code 255 failure on net11.0 iOS builds maui#34798 (comment)