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/concat-map/example/map.js

@@ -0,0 +1,6 @@
+var concatMap = require('../');
+var xs = [ 1, 2, 3, 4, 5, 6 ];
+var ys = concatMap(xs, function (x) {
+    return x % 2 ? [ x - 0.1, x, x + 0.1 ] : [];
+});
+console.dir(ys);