runSerial.sh: Autodetect RXTX library location

This fixes a crash on start on my Fedora box (stacktrace below).
Also, give a proper error message if the library cannot be found.

Former crash on start:
$ ./runSerial.sh
Exception in thread "main" java.lang.NoClassDefFoundError: gnu/io/NoSuchPortException
	at java.lang.Class.getDeclaredMethods0(Native Method)
	at java.lang.Class.privateGetDeclaredMethods(Class.java:2531)
	at java.lang.Class.getMethod0(Class.java:2774)
	at java.lang.Class.getMethod(Class.java:1663)
	at sun.launcher.LauncherHelper.getMainMethod(LauncherHelper.java:494)
	at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:486)
...