is duplicated by:

• SR-9775 swift build fails with Xcode 10.2 toolchain, but is ok with 10.1 toolchain
• SR-10205 SPM fails to resolve dependencies with Swift 5

relates to:

• SR-4072 Better response rather than "unsatisfiable"
• SR-4236 Unsatisfiable dependencies error should mention pinning

Issue Description:

Currently if SwiftPM is not able to find an eligible package version of a dependency because the swift-tools-version is too high, the error you get is just:
error: unsatisfiable
We should instead emit an error that:

1. Indicates that the problem is the swift-tools-version is higher than the current tools version
2. Tells you which dependency can't be resolved

Steps to reproduce:

1. Create a package whose swift-tools-version for all tags is 4.0
2. Create another package that has a dependency on the first package
3. Try to build the second package, using swift 3.1 tools