[bugfix] fix media create error not being checked

[tsmethurst]

Description

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This PR fixes the errWithCode not being checked while processing media via the client API endpoint, leading to null pointer panics when trying to upload bad media.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas. -- N/A
• I/we have made any necessary changes to documentation. -- N/A
• I/we have added tests that cover new code. -- N/A
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.