'file not found' error due to missing .k extension

[DennisMitchell]

Since #507 got closed, Kona script files need a .k extension, and ./k <basename> automatically appends the extension to basename if it's not present.

For files without the extension, this causes a rather confusing error message to be printed.

$ ./k helloworld
helloworld: file not found
$ ls helloworld
helloworld

One solution would be to print the modified filename instead.

$ ./k helloworld
helloworld.k: file not found