flycheck-pos-tip fails on terminals

[StreakyCobra]

With the current implementation, flycheck-pos-tip throws an error when emacs is run within a terminal.

Flycheck error display error: (error Window system frame should be used)

It would be nice to handle this case, maybe with a fallback in the echo area when there is no window system.

See syl20bnr/spacemacs#4111 for reference.

[swsnr]

We'll fall back to the built-in echo area display on non-graphical display. The fallback will be customisable.

[StreakyCobra]

👍 Many thanks for your support :-)

[cpitclaudel]

@lunaryorn I can make a PR for this one (this week-end) if you want :)

[swsnr]

@cpitclaudel That would be great!