Fix the duplicated GLTFLoadException (generate a TEXCOORD_4 error instead of a warning)

[EricBeetsOfficial-Opuscope]

As described in issue #499 and #577.
Delete a duplicated GLTFLoadException which emit an error instead of a warning when there is TEXCOORD_4 attribute (during runtime import).

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Eric seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}