Fix returned eventsource status code when not found

[aarranz]

Standard states that the correct status code is 204 instead of the normal 404.

[coveralls]

Pull Request Test Coverage Report for Build 13

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 89.103%

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Totals
Change from base Build 11:	0.04%
Covered Lines:	232
Relevant Lines:	251

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💛 - Coveralls