fix: improve input validation and error reporting

Validate the nice value is an integer in [-20, 19] at startup rather
than letting nice(1) fail with a cryptic error at encode time.

In encode_one, move the "Processing" message after the empty-string
guard so it is not printed for blank lines, include the filename in the
"Not found" error, and add a readability check so unreadable files are
rejected with a clear message before ffprobe is invoked.

Validate that the encode file (-f) exists before entering the read
loop, exiting with a usage error rather than a silent empty run.

0 files changed, 0 insertions, 0 deletions