Emit compile error when no runtime feature specified

[mzeitlin11]

Inspired by an error I saw when using https://github.com/launchbadge/sqlx. Hopefully gives a bit quicker feedback about how to solve an otherwise giant compilation error when forgetting to specify a runtime feature, though could still be improved as mentioned in the comment.

[codecov]

Codecov Report

Merging #282 (af1aef5) into master (9da5cd9) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #282   +/-   ##
======================================
  Coverage    5.36%   5.36%           
======================================
  Files         150     150           
  Lines       14526   14526           
======================================
  Hits          780     780           
  Misses      13746   13746           

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[arlyon]

Lovely, TY! I have a PR to fix a longstanding bug #246 which I will request your feedback on if you'd like. I know I said we should sync real quick and talk about plans (since you've decided to stick around! :) ) so if you want to you can book a slot here

https://usemotion.com/meet/alexander-lyon/meeting

Merging!

[mzeitlin11]

Lovely, TY! I have a PR to fix a longstanding bug #246 which I will request your feedback on if you'd like. I know I said we should sync real quick and talk about plans (since you've decided to stick around! :) ) so if you want to you can book a slot here

Thanks, will book one! (Though please don't have too high hopes time I can spend on this tends to be inconsistent :))