date --rfc-3339 foo error should be managed properly

[sylvestre]

Found with the fuzzer introduced in #4389

$ ./target/debug/coreutils date --rfc-3339 foo      
thread 'main' panicked at 'Invalid format: foo', src/uu/date/src/date.rs:136:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

with GNU:

date --rfc-3339 foo
date: invalid argument 'foo' for '--rfc-3339'
Valid arguments are:
  - 'date'
  - 'seconds'
  - 'ns'
Try 'date --help' for more information.

Similar too #4499 . cc @shanmukhateja if you want to do it here too

[tertsdiepraam]

I think you meant: cc @shanmukhateja :)

[sylvestre]

ah ah, oups :)
I will fix it

[shanmukhateja]

Yup I'm up for it.