Crash dispatching desktop notifications in Linux environments without GUI

[lelandjansen]

The CLI crashes with a stack overflow when trying to dispatch desktop notifications in Linux environments without a GUI (e.g. in CI).

Introduced in #24.

# cargo run -- echo hello
    Finished dev [unoptimized + debuginfo] target(s) in 0.38s
     Running `target/debug/diditrun echo hello`
hello

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
Aborted (core dumped)

To get around this (temporarily,) we use a dummy DesktopNotifier for Linux tests.

Things that need to get fixed:

• Do not crash (at least return a Result and print an error).
• Detect that we are not running in a GUI environment and fail early with an error (e.g. "cannot display desktop notifications in a non-GUI environment"). The user should explicitly pass the --no-desktop flag or override the error with the --no-validate flag.