search_next_impl: don't panic on invalid regex

[pppKin]

This should resolve #738

[archseer]

While this fixes the issue, we probably shouldn't store searches that are invalid in the register altogether. Prompt closure would need a result value so we're able to signal errors better.

[pppKin]

Ah, that makes sense, that would make it easier for everyone else using `RegexPrompt`.

[pppKin]

Or perhaps we still store the invalid regex? I know vim does this(perhaps other editors too). It could be useful to use up arrow key to retrieve search histories, modify a bit and search again.

[archseer]

Good point, let's merge this 👍🏻

[pppKin]

Since #651 will introduce a unified callback in regex_prompt which would make this PR a bit cleaner and tidier, let's wait till #651 is finished and merged;)

[archseer]

I think this is okay to merge, the callback change won't really affect this PR (register setting was refactored on master and happens inside Prompt now)

[pppKin]

Oh I see. Should be fine to merge than. If any further improvement is required we'll just open a new issue:)