Print helpful message when requested version doesn't have available download for platform

[patrickfreed]

Some versions of Swift are not available on certain platforms. For example, Ubuntu 22.04 only has downloads available for 5.7+. Right now, swiftly prints something like "Swift 5.6.3 doesn't exist", which isn't the most helpful message. Instead, it should be more clear about the fact that the given version does exist, but is just not available for the user's platform.