parse_event now distinguishes between invalid and incomplete input

When parsing an event, if input is expected and not available,
parse_event will return an Error with ErrorKind WouldBlock.
All other error conditions are unaffected.
6 jobs from distinguish-partial-matches in 3 minutes and 27 seconds (queued for 2 seconds)
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passed #12598
build:beta

00:01:26

passed #12599
build:nightly

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passed #12597
build:stable

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  Test
passed #12601
test:beta

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passed #12602
test:nightly

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passed #12600
test:stable

00:01:52