Error MSB4012 could be more helpful

In the spirit of https://twitter.com/GregorySchier/status/732830868562182144, this could be improved:

`error MSB4012: The expression "o:\msbuild\bin\Debug-NetCore\Windows_NT_Deployment\\@(ResolvedRuntimeFiles->'%(DestinationSubDirectory)')" cannot be used in this context. Item lists cannot be concatenated with other strings where an item list is expected. Use a semicolon to separate multiple item lists.`

"Use a semicolon" is a good suggestion (and should probably be the only one if there's no item transform involved), but there's another common case where you can run into this: when the solution is to move the initial string _inside_ the item transform. We could suggest that rewriting as well:

`error MSB4012: The expression "{0}" cannot be used in this context. Item lists cannot be concatenated with other strings where an item list is expected. Use a semicolon to separate multiple item lists, or move {1} inside the item transformation to prefix every transformation output with the string.`