pcvalidate panics if the file argument doesn't exist

[bfabio]

❯ ./pcvalidate/pcvalidate foo
open /home/fabio/dev/publiccode-parser-go/foo: no such file or directory
panic: interface conversion: error is *fs.PathError, not publiccode.ValidationResults

goroutine 1 [running]:
main.hasValidationErrors(...)
	/home/fabio/dev/publiccode-parser-go/pcvalidate/pcvalidate.go:109
main.main()
	/home/fabio/dev/publiccode-parser-go/pcvalidate/pcvalidate.go:96 +0x568