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/exec-sh/example/example.js

@@ -0,0 +1,17 @@
+var execSh = require('../')
+
+// run interactive bash shell
+execSh('echo ola && bash', { cwd: '/home' }, function (err) {
+  if (err) {
+    console.log('Exit code: ', err.code)
+    return
+  }
+
+  // collect streams output
+  execSh(['bash -c id', 'echo olaola >&2'], true,
+    function (err, stdout, stderr) {
+      console.log('error: ', err)
+      console.log('stdout: ', stdout)
+      console.log('stderr: ', stderr)
+    })
+})