Add error field name to multipart response

[omerzi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.
• I labeled this pull request with one of the following: 'breaking change', 'new feature', 'bug', or 'ignore for release'

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This pull request enhances error handling when writing files to a multipart/form-data stream. Once an error occurs, the streaming stops, and the error will be written into a new part with the field name "error".