Implemente file-exists and try-catch

[Yuexi Ma]

1. File operation:

(file-exists? sth)

2. try-catch method

Without both of these it seems hard to write scripts that read image from file, for example, the graphics example code: chance

[Doug Blank]

There is try/catch. See: http://wiki.roboteducation.org/Calico_Scheme#Exceptions