Fix unused error value.

[jsirois]

Previously we'd see:

   Compiling ui v0.0.1 (/home/jsirois/dev/pantsbuild/pants/src/rust/engine/ui)
warning: unused `std::result::Result` which must be used
   --> ui/src/display.rs:178:5
    |
178 |     self.flush();
    |     ^^^^^^^^^^^^^
    |
    = note: #[warn(unused_must_use)] on by default
    = note: this `Result` may be an `Err` variant, which should be handled

[blorente]

Thanks! Sorry I couldn't fix it earlier, but got sidetracked.

[stuhood]

Looks like this hit #6829. Sorry about that: will take a look today after making progress on some other stuff.