download TODO comment: handle collision when writing solution files

[jdsutherland]

In effort to resolve TODO comments, I'm in the process of promoting them to issues.

cli/cmd/download.go

Lines 198 to 199 in a86f829

	// TODO: if there's a collision, interactively resolve (show diff, ask if overwrite).
	// TODO: handle --force flag to overwrite without asking.

It seems like #457 is related. Is this still pending further info?

[kytrinyx]

I've come to the conclusion that I don't want to make the CLI interactive if we can avoid it (at least not until we've worked out all the existing kinks in a non-interactive way).

I think the solution here would be to display an error message if there's a collision, including a pointer at how you could resolve it.

[haguro]

With the premiss of an interactive CLI out of scope for the time being, and the --force flag having been added in #979. I believe this should count as resolved?