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/node-notifier/vendor/mac.noindex/terminal-notifier.app/Contents/Resources/en.lproj/Credits.rtf

@@ -0,0 +1,29 @@
+{\rtf0\ansi{\fonttbl\f0\fswiss Helvetica;}
+{\colortbl;\red255\green255\blue255;}
+\paperw9840\paperh8400
+\pard\tx560\tx1120\tx1680\tx2240\tx2800\tx3360\tx3920\tx4480\tx5040\tx5600\tx6160\tx6720\ql\qnatural
+
+\f0\b\fs24 \cf0 Engineering:
+\b0 \
+	Some people\
+\
+
+\b Human Interface Design:
+\b0 \
+	Some other people\
+\
+
+\b Testing:
+\b0 \
+	Hopefully not nobody\
+\
+
+\b Documentation:
+\b0 \
+	Whoever\
+\
+
+\b With special thanks to:
+\b0 \
+	Mom\
+}