Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/babel-jest/tsconfig.json

@@ -0,0 +1,12 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "build"
+  },
+  // TODO: include `babel-preset-jest` if it's ever in TS even though we don't care about its types
+  "references": [
+    {"path": "../jest-transform"},
+    {"path": "../jest-types"}
+  ]
+}