Throw error if debug.yaml not found

Sources/swift-test/Error.swift

@@ -0,0 +1,22 @@
+/*
+This source file is part of the Swift.org open source project
+
+Copyright 2015 - 2016 Apple Inc. and the Swift project authors
+Licensed under Apache License v2.0 with Runtime Library Exception
+
+See http://swift.org/LICENSE.txt for license information
+See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+enum Error: ErrorType {
+    case DebugYAMLNotFound
+}
+
+extension Error: CustomStringConvertible {
+    var description: String {
+        switch self {
+        case .DebugYAMLNotFound:
+            return "build the package using `swift build` before running tests"
+        }
+    }
+}

Sources/swift-test/main.swift

@@ -24,6 +24,8 @@ do {
     setenv("SPM_INSTALL_PATH", dir.build, 0)
 
     let yamlPath = Path.join(dir.build, "debug.yaml")
+    if !yamlPath.exists { throw Error.DebugYAMLNotFound }
+
     try build(YAMLPath: yamlPath, target: "test")
     let success = test(dir.build, "debug")
     exit(success ? 0 : 1)