[check-licenses] Validate license name in allowlist add

Update AddAllowlistEntry in allowlist.go to reject unknown license
names. If findLicenseCategory returns "Uncategorized", it means the
license has not been approved or structured under the asset tree yet.
The tool now returns an error immediately to prevent typos from silently
generating dead configuration file noise. Added unit test for unknown
license name validation.

Change-Id: I0cfd0eff36773cd64fd0c459c30c1fd0f6f44221
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1602052
Reviewed-by: Jiaming Li <lijiaming@google.com>
Commit-Queue: Jerry Belton <jcecil@google.com>