Add command to FAIL and WHAT.

Also constify some things that should be const.

Fixes #64.  Unit test fixes forthcoming.
Fix unit tests for CommandResult.
Bonus: Fix missing space in couldn't-open error.

Ronseal
Use insert, not emplace.
Update command references for FAIL/WHAT change.
Merge pull request #65 from UniversityRadioYork/mw-fail-what-command

Add command to FAIL and WHAT.