Improve error handling when a key is missing

[lewing]

This pr is to avoid

.packages\microsoft.dotnet.arcade.sdk\8.0.0-beta.23428.2\tools\InstallDotNetCore.targets(15,5): error MSB4018: (NETCORE_ENGINEERING_TELEMETRY=Restore) The "InstallDotNetCore" task failed unexpectedly.
System.InvalidOperationException: Sequence contains no elements
   at System.Linq.ThrowHelper.ThrowNoElementsException()
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
   at Microsoft.DotNet.Arcade.Sdk.InstallDotNetCore.Execute() in /_/src/Microsoft.DotNet.Arcade.Sdk/src/InstallDotNetCore.cs:line 110
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask)

when the property doesn't exist.

cc @mmitche