detect missing RLS component and give a useful message

[nrc]

If the rls-preview component is missing (which happens fairly often), we should advise the user that this is the case, explain why it happens, and suggest a remedy (downgrade or use beta/stable).

[ejmahler]

This should supersede #203 imo

[mattico]

Even better, suggest and install the previous nightly which works: https://www.reddit.com/r/rust/comments/7sds1a/atom_iderust_011_handling_nightly_does_not/

[Xanewok]

FWIW during the component installation we now present the rustup component add rls in the task view, along with the output. This means that the user sees what's wrong, however it might be good to follow Atom in its footsteps and suggest using the previous nightly which includes the component.