janet -c source with missing output files should give informative error message

[kredati]

This is my first issue; apologies if I get style or substance wrong. I absolutely adore Janet.

Desired behaviour (if not exactly expected):

$ janet -c source.janet 
> Error: expected output image path`

Actual behaviour:

$ janet -c source.janet
> error: expected integer key for array in range [0, 2), got 2
  in c-switch [boot.janet] (tailcall) on line 4066, column 18
  in cli-main [boot.janet] on line 4102, column 13

I feel like giving an internal stack trace on a user error like this is not ideal.

Alternative, possibly desired behaviour, is to use the source filename for the new image file:

$ janet -c source.janet
> Successfully builds source.jimage

[bakpakin]

I think this would be a good change, including the default argument