Add new nullable_return_is_error configuration for return types

[sdroege]

This allows converting an Option into an Result<T, glib::BoolError>,
which in case of Option signalling error allows for more ergonomic error
handling code.

---

CC @EPashkin @GuillaumeGomez

[EPashkin]

Also new warning than IMHO better ignored by "#[allow(clippy::large_enum_variant))".

warning: large size difference between variants
  --> src\chunk\chunk.rs:23:5
   |
23 | /     FfiCallConversion {
24 | |         ret: return_value::Info,
25 | |         array_length_name: Option<String>,
26 | |         call: Box<Chunk>,
27 | |     },
   | |_____^
   |
   = note: `#[warn(clippy::large_enum_variant)]` on by default
help: consider boxing the large fields to reduce the total size of the enum
  --> src\chunk\chunk.rs:23:5
   |
23 | /     FfiCallConversion {
24 | |         ret: return_value::Info,
25 | |         array_length_name: Option<String>,
26 | |         call: Box<Chunk>,
27 | |     },
   | |_____^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant

[sdroege]

Fixed, and also some other clippy warnings. Thanks for the review

[EPashkin]

@sdroege Big thanks