Write a shell script that will rename all the files in a directory. The first argument is a base name, second argument is a file extension. If the script is run as: $ renumber 25thAnniversary jpeg then the resulting files should have names like: 25thAnniversary001.jpeg, 25thAnniversary002.jpeg, 25thAnniversary003.jpeg, etc.

Before renaming a file, make sure that a file with the new name does not already exist. If one does, handle the exception appropriately.