Remove the `{ffmpeg}` variable from the default configuration, and its use
from the job compiler scripts. Now that the Worker can find its bundled
FFmpeg, it's no longer needed to configure its location on the Manager.
Worker will now try one of the following paths, relative to the flamenco-worker
executable, in order to find FFmpeg. If they cannot be found, `$PATH` is
searched for FFmpeg.
- `tools/ffmpeg-$GOOS-$GOARCH`
- `tools/ffmpeg-$GOOS`
- `tools/ffmpeg`
On Windows these paths will have a `.exe` suffix appended. `$GOOS` is the
operating system, like "linux", "darwin", "windows", etc. `$GOARCH` is the
architecture, like "amd64", "386", etc.
`os.IsNotExist()` is from before `errors.Is()` existed. The latter is the
recommended approach, as it also recognised wrapped errors.
No functional changes, except for recognising more cases of "does not
exist" errors as such.
Introduce `ParameterMissingError` and `ParameterInvalidError` structs, to
be returned from command executors. These replace free-form `fmt.Errorf()`
style errors.
Fix the FFmpeg unit test on Windows, by:
- Having actual input files (otherwise the input-glob-creation-function
errors out), and
- ensuring the cleanup function is always run, and
- testing for the right CLI arguments.
