when the end-user installs on an arch/os that doesn't have packaged binaries, a nice message should be given

`elm-platform` current uses a message like this:

`var errorMessage = "Unfortunately, there are no elm-format " + packageInfo.version + " binaries available for your operating system and architecture.\n\nIf you would like to build elm-format from source, there are instructions at https://github.com/avh4/elm-format#building-from-source\n";`

Start by locating the code path that handles an unavailable binary and compare it with the `elm-platform` message shown in the issue. Update the user-facing error so it clearly explains that no packaged binary exists for the operating system and architecture and points to the source-building instructions; verify the unavailable-platform path displays it.

Written by the indexing model from the issue text.