Make error handler print #full_message if available

[denisdefreyne]

Detailed description

If an error implements #full_message, it will be printed after the error detail. This makes the nanoc-dart-sass errors much more interesting:

Captain! We’ve been hit!

Sass::CompileError: Undefined mixin.

Error: Undefined mixin.
  ╷
9 │   @include oops;
  │   ^^^^^^^^^^^^^
  ╵
  nanoc:/assets/style/_2021q4_common.* 9:3  @use
  - 7:1                                     root stylesheet

Current item: /assets/style/2021q4.scss (:default representation)

To do

• Tests

Related issues

n/a

[DivineDominion]

Tested in my project, looks great 👍

[denisdefreyne]

This is released in 4.12.16 now.