added hidden nonexaustive variant

[tinaun]

fixes #182

i first tried to add the variant inside error_chain_processed!, but that failed with a weird macro error:

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
  --> src\quick_error.rs:98:20
   |
98 |             enum [$( $(#[$emeta])* => $eitem $(( $($etyp),* ))* )*
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

so i had to drop down to quick_error and impl'd it there. seems to work ok.

[Yamakaky]

Could you update the changelog?

[Yamakaky]

Also, add "Fixes #182" in the commit body.

[faern]

Bump. Feedback has been fixed already :)

[faern]

@tinaun Rebase it on latest master maybe?

[Yamakaky]

Yes

[Yamakaky]

Sorry for the delay: could you rebase again?

[Yamakaky]

Manually merged, thanks!