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/exit/test/fixtures/10-stdout.txt

@@ -0,0 +1,10 @@
+stdout 0
+stdout 1
+stdout 2
+stdout 3
+stdout 4
+stdout 5
+stdout 6
+stdout 7
+stdout 8
+stdout 9