unwrap the Option to access the backtrace

[scampi]

On master running cargo test --all fails with:

error[E0599]: no method named `is_empty` found for type `std::option::Option<&failure::Backtrace>` in the current scope
  --> failure_derive/tests/wraps.rs:61:29
   |
61 |     assert!(err.backtrace().is_empty());
   |                             ^^^^^^^^

error[E0599]: no method named `is_empty` found for type `std::option::Option<&failure::Backtrace>` in the current scope
  --> failure_derive/tests/wraps.rs:62:32
   |
62 |     assert_eq!(err.backtrace().is_empty(), err.backtrace().to_string().trim().is_empty());
   |                                ^^^^^^^^

error[E0599]: no method named `to_string` found for type `std::option::Option<&failure::Backtrace>` in the current scope
  --> failure_derive/tests/wraps.rs:62:60
   |
62 |     assert_eq!(err.backtrace().is_empty(), err.backtrace().to_string().trim().is_empty());
   |                                                            ^^^^^^^^^
   |
   = note: the method `to_string` exists but the following trait bounds were not satisfied:
           `std::option::Option<&failure::Backtrace> : std::string::ToString`

error[E0599]: no method named `is_empty` found for type `std::option::Option<&failure::Backtrace>` in the current scope
  --> failure_derive/tests/wraps.rs:96:29
   |
96 |     assert!(err.backtrace().is_empty());
   |                             ^^^^^^^^

error[E0599]: no method named `is_empty` found for type `std::option::Option<&failure::Backtrace>` in the current scope
  --> failure_derive/tests/wraps.rs:97:32
   |
97 |     assert_eq!(err.backtrace().is_empty(), err.backtrace().to_string().trim().is_empty());
   |                                ^^^^^^^^

error[E0599]: no method named `to_string` found for type `std::option::Option<&failure::Backtrace>` in the current scope
  --> failure_derive/tests/wraps.rs:97:60
   |
97 |     assert_eq!(err.backtrace().is_empty(), err.backtrace().to_string().trim().is_empty());
   |                                                            ^^^^^^^^^
   |
   = note: the method `to_string` exists but the following trait bounds were not satisfied:
           `std::option::Option<&failure::Backtrace> : std::string::ToString`