TSCBasic: repair the Windows build after apple/swift-package-manager#2925
TSCBasic: repair the Windows build after apple/swift-package-manager#2925

The `ProcessResult.Error` conformance to `CustomStringConvertible` was
ignored on the Windows target accidentally.  Adjust this to repair the
build.

apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core