Submitting with no upload file causes 500 error

[brittainhard]

Need to add some logic to ensure file exists.

[martinholmer]

Using TaxBrain 1.0.1 on 2017-08-16, this bug persists.

Going to the file-upload page and immediately clicking on the `Show me the results!' button, produces this:

[hdoupe]

Closed via #641

[martinholmer]

Issue fixed in TaxBrain 1.0.3. @hdoupe, thanks for the work on this.